One evaporator does not cool, the others do

Troubleshooting · Applies to: Cold rooms with several evaporators

Troubleshoot when one cold room or cabinet gets warm while the others on the same unit cool: controls, solenoid valve, ice, fan and expansion valve.

Last checked: October 25, 2026 Field mode: large buttons, questions first and the screen kept on.

When only one evaporator is warm the fault is almost always in that branch, because the unit supplies the others. Danfoss mentions that a solenoid valve can open partially because of too low a voltage, a wrong coil, dirt or a damaged armature tube ^[1].

Work on the refrigerant circuit may only be carried out by certified personnel. Isolate the power before working on electrical parts and take particular care with high pressures (CO2) and flammable refrigerants (A2L and A3). Always follow the manufacturer's instructions.

Confirm the fault first

Measure the air temperature in and out of the evaporator and the temperature before and after the solenoid valve. Check that the room thermostat calls for cooling and that the coil gets voltage, that the evaporator fans run, that all shut-off valves in the branch are open, and that the evaporator is not in defrost.

📏 Measurement guide: calculate superheat and subcooling 🔢 Alarm and fault codes

Flowchart

Causes, most likely first

The order is approximate and based on common faults in the field. Always measure before replacing anything.

1
Flowchart A
The control signal is missing: thermostat, sensor, relay or cable.
Remedy: Follow the signal from the thermostat to the coil and measure the voltage. Replace a faulty relay or sensor.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Trace the control and safety chain
When a compressor or fan does not get a start signal, follow the control voltage through the safety chain in the wiring diagram to the point where it disappears. Live measurements in an open panel are for qualified personnel only.
1. Get the wiring diagram. Find the diagram for the unit and mark the path from the control supply through fuses, safety devices (HP/LP switches, motor protection, thermal protection) and controller outputs to the contactor coil. Use a CAT III/IV meter with insulated probes.
  ✓ Check: The diagram matches the unit's model and serial plate, and you know the order of the devices.
2. Check the control supply. Measure the voltage at the secondary of the control transformer or power supply and check the control fuses.
  ✓ Check: The rated control voltage (e.g. 24 V AC or 230 V AC) is present and the fuses are intact.
3. Follow the chain device by device. Measure from each terminal in the chain to neutral or control common, in the order of the diagram. Never bridge a safety switch to make the system run.
  ✓ Check: You find the point where voltage is present on the input but 0 V on the output: that device is open.
4. Find out why the device is open. Read the device: is the HP switch tripped and does it have a manual reset, is the motor protection tripped, is a door or flow switch open? Check the cause (pressure, current, temperature) before resetting.
  ✓ Check: The cause of the trip is known, not only the device.
5. Check the controller output and relays. With a call for operation active, measure the output at the controller relay terminal. A relay that is energised but has no voltage on its output, or that stays closed without a call, is faulty.
  ✓ Check: The output follows the call: voltage when the controller calls and none when it does not.
6. Repair and test. Isolate and lock off before replacing a part or tightening terminals. Restore power, reset and check that the chain closes and the contactor pulls in.
  ✓ Check: The system starts on call and stops when each safety device opens.
Check a solenoid valve (liquid line or hot gas)
A solenoid valve that does not open starves the evaporator (low pressure, warm room); one that does not close lets refrigerant through when it should not (pump-down fails, flooding, or hot gas leaking into the evaporator). Check the coil first, then the valve itself.
1. Find out whether the valve should be open. Read in the controller or the wiring diagram when the valve is energised (for example with cooling demand, during pump-down or during hot gas defrost). Compare with what the system does right now.
  ✓ Check: You know whether the valve should be open or closed at this moment.
2. Check that the coil gets voltage. Measure the voltage across the coil terminals when the valve should be open. Hold a steel screwdriver blade near (not touching) the top of the coil: a live coil pulls it slightly. Check that the coil voltage on its label matches the supply.
  ✓ Check: The coil has the rated voltage (label) when the valve should be open; if not, the fault is upstream: thermostat, controller output, contactor, fuse or wiring.
3. Check the coil itself. Isolate and lock off, prove dead, and disconnect the coil. Measure its resistance and compare with the manufacturer's value; look for a burnt smell, discolouration or a cracked coil. Make sure the coil is the right type for the valve body and sits fully down on the armature tube with its nut tight.
  ✓ Check: Coil resistance matches the data (an open circuit or a very low value means a faulty coil), and the coil is the right one and correctly fitted.
4. Check that the valve opens. With the coil energised and the system running, measure the temperature on both sides of the valve with clamp thermometers. An open valve has (almost) the same liquid temperature on both sides; a clear temperature drop or frost after the valve means it is not opening fully or is restricted. Check the flow direction arrow on the valve body.
  ✓ Check: Little or no temperature difference across the valve when it should be open, and it is fitted in the direction of the arrow.
5. Check that the valve closes. De-energise the coil (or let the controller close it) and watch what happens: on a pump-down system the suction pressure must fall and the compressor must stop on the low pressure switch and stay off; a hot gas valve must leave the line after it cool during the freeze or cooling cycle.
  ✓ Check: The system pumps down and stays down, or the line after a hot gas valve stays cool; a valve that leaks through must be repaired or replaced.
6. Repair or replace. A faulty coil can be changed without opening the circuit. If the valve body leaks or sticks, opening the circuit is work for certified personnel: recover the refrigerant, replace the valve (or its internal parts) with nitrogen flowing while brazing, pressure test with dry nitrogen, evacuate and charge by weight. Protect the valve from heat while brazing as the manufacturer describes.
  ✓ Check: The valve opens and closes as it should, there is no leak at the new joints, and the work is recorded in the logbook where the refrigerant rules require it.
2
Flowchart B
The solenoid valve opens only partially: too low a voltage, wrong coil, dirt or a damaged armature tube.
Remedy: Measure the coil voltage, check the coil marking and clean or replace the valve insert ^[1].
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a solenoid valve (liquid line or hot gas)
A solenoid valve that does not open starves the evaporator (low pressure, warm room); one that does not close lets refrigerant through when it should not (pump-down fails, flooding, or hot gas leaking into the evaporator). Check the coil first, then the valve itself.
1. Find out whether the valve should be open. Read in the controller or the wiring diagram when the valve is energised (for example with cooling demand, during pump-down or during hot gas defrost). Compare with what the system does right now.
  ✓ Check: You know whether the valve should be open or closed at this moment.
2. Check that the coil gets voltage. Measure the voltage across the coil terminals when the valve should be open. Hold a steel screwdriver blade near (not touching) the top of the coil: a live coil pulls it slightly. Check that the coil voltage on its label matches the supply.
  ✓ Check: The coil has the rated voltage (label) when the valve should be open; if not, the fault is upstream: thermostat, controller output, contactor, fuse or wiring.
3. Check the coil itself. Isolate and lock off, prove dead, and disconnect the coil. Measure its resistance and compare with the manufacturer's value; look for a burnt smell, discolouration or a cracked coil. Make sure the coil is the right type for the valve body and sits fully down on the armature tube with its nut tight.
  ✓ Check: Coil resistance matches the data (an open circuit or a very low value means a faulty coil), and the coil is the right one and correctly fitted.
4. Check that the valve opens. With the coil energised and the system running, measure the temperature on both sides of the valve with clamp thermometers. An open valve has (almost) the same liquid temperature on both sides; a clear temperature drop or frost after the valve means it is not opening fully or is restricted. Check the flow direction arrow on the valve body.
  ✓ Check: Little or no temperature difference across the valve when it should be open, and it is fitted in the direction of the arrow.
5. Check that the valve closes. De-energise the coil (or let the controller close it) and watch what happens: on a pump-down system the suction pressure must fall and the compressor must stop on the low pressure switch and stay off; a hot gas valve must leave the line after it cool during the freeze or cooling cycle.
  ✓ Check: The system pumps down and stays down, or the line after a hot gas valve stays cool; a valve that leaks through must be repaired or replaced.
6. Repair or replace. A faulty coil can be changed without opening the circuit. If the valve body leaks or sticks, opening the circuit is work for certified personnel: recover the refrigerant, replace the valve (or its internal parts) with nitrogen flowing while brazing, pressure test with dry nitrogen, evacuate and charge by weight. Protect the valve from heat while brazing as the manufacturer describes.
  ✓ Check: The valve opens and closes as it should, there is no leak at the new joints, and the work is recorded in the logbook where the refrigerant rules require it.
3
The evaporator fan has stopped: motor, fuse or fan control.
Remedy: Measure voltage and current on the fan motor.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a condenser or evaporator fan motor
A fan that stops, runs slowly or turns the wrong way gives high pressure on the condenser side, or low pressure and ice on the evaporator side. Check the supply, the control signal, the motor and the fan itself.
1. Look and listen with the unit running. Watch the fan from a safe distance: does it run, at what speed, and in the right direction (air through the coil the way the arrow on the housing or the manual shows)? Listen for bearing noise and look for ice, dirt or a damaged blade.
  ✓ Check: You know whether the fan is stopped, slow, noisy or turning the wrong way.
2. Check the supply and the control signal. Measure the voltage at the motor terminals or the EC fan connector with the fan called for. For an EC fan also measure the control signal (0–10 V or PWM) and read any status lamp or fault code on the fan electronics. Check fan delay settings and thermostats in the controller: a fan that is held off on purpose is not faulty.
  ✓ Check: Supply voltage is within the rated range on the nameplate, and the control signal asks for the fan to run; otherwise the fault is upstream (contactor, fuse, controller output, wiring).
3. Isolate, lock off and make sure the fan cannot start. Switch off and lock the isolator, prove the circuit dead with a tested two-pole tester, and wait for the fan to stop completely. Remember that an EC fan can restart on its own when the signal returns, and that a fan wind-milling in a draught can generate voltage.
  ✓ Check: 0 V on the motor terminals and the blades are blocked or stationary.
4. Turn the fan by hand and check it mechanically. Turn the impeller by hand: it should spin freely and evenly, without scraping, axial play or a rough bearing. Check that the blades are tight on the shaft and not cracked, and that the guard and the coil are free of ice, dirt and loose objects.
  ✓ Check: The fan turns freely and quietly and the blades are undamaged; a stiff or rough motor must be replaced.
5. Measure the motor (AC motors). With the motor disconnected, measure the winding resistance between the terminals and compare the windings with each other and with the data sheet; test the run capacitor (see the capacitor guide). Measure the insulation resistance to earth with an insulation tester – never with the fan electronics or an EC motor connected. Check that the motor's internal thermal protection is closed when the motor is cold.
  ✓ Check: Windings are equal where they should be and match the data sheet, insulation resistance is well above the limit your electrical rules require, and the thermal protection is closed when cold.
6. Restore, run and measure the current. Reconnect from your photo, fit guards and covers, remove the lock and start. Measure the running current on each phase and compare it with the nameplate, and check the direction and the air flow.
  ✓ Check: The fan runs smoothly in the right direction, the current is at or below the nameplate value and the pressures settle (head pressure falls on a condenser, suction pressure rises and ice clears on an evaporator).
4
Flowchart C
The evaporator is iced up.
Remedy: Troubleshoot the iced evaporator.
Continue with: Evaporator keeps icing up →
5
Flowchart D
The expansion valve underfeeds: blocked strainer, moisture freezing in the valve, a bulb that has lost its charge or a wrong setting.
Remedy: Measure superheat. Clean the strainer and check the bulb and the setting. Ice at the valve inlet points to moisture: replace the filter drier.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Measure superheat
Superheat shows whether the evaporator is fed with the right amount of refrigerant: too high starves the evaporator, too low risks liquid reaching the compressor. Measure at the evaporator outlet (for the expansion valve) and at the compressor (to protect it).
1. Let the system stabilise. Run at normal load for at least 10–15 minutes, not during or right after defrost. Wear goggles and gloves (cold pipes can cause frostbite); with A2L/A3 refrigerants keep ignition sources away.
  ✓ Check: Suction pressure and pipe temperature no longer drift.
2. Read the suction pressure. Connect the gauge to a port at the evaporator outlet if there is one; a port at the compressor shows a lower pressure because of the pressure drop in the suction line. Select the correct refrigerant.
  ✓ Check: The gauge shows a stable pressure, and you know where it was measured.
3. Measure the pipe temperature. Clamp a pipe thermometer on clean bare pipe next to the expansion valve bulb (evaporator superheat) and on the suction line about 15–30 cm before the compressor (compressor superheat), insulated from the surrounding air.
  ✓ Check: Both temperatures are stable.
4. Calculate superheat. Convert the pressure to the saturation temperature; for zeotropic blends use the dew point. Superheat = measured pipe temperature − saturation temperature.
  ✓ Check: The result is positive and is compared with the manufacturer's target; for a TXV at the evaporator outlet it is often around 5–8 K.
5. Interpret before adjusting. High superheat together with bubbles in the sight glass or low subcooling points to low charge or a restriction, not a valve setting. Very low compressor superheat means a risk of liquid floodback. Check charge, sight glass and filter drier before touching the valve.
  ✓ Check: The cause is supported by at least one other reading.
6. Adjust only when the cause is the valve. Adjust a TXV in small steps (about a quarter turn) and wait 15–20 minutes between steps; on an electronic valve change the setpoint in the controller. Record the values before and after.
  ✓ Check: Superheat settles at the target and stays stable during a full cycle.
Check a thermostatic expansion valve
A TXV that sticks, has lost its bulb charge, has a blocked strainer or ice in the orifice gives high superheat and poor cooling. Check the valve's response first; open the circuit only when you know the valve must be cleaned or replaced.
1. Measure the starting point. Connect gauges to the suction side and a clamp thermometer at the bulb position on the evaporator outlet. Let the system run stable and calculate the superheat.
  ✓ Check: Suction pressure, pipe temperature and superheat are noted with the system running stable.
2. Warm the bulb. Loosen the bulb and warm it gently in your hand or in warm water (no flame or heat gun close to it). The valve should open.
  ✓ Check: The suction pressure rises and the superheat falls within a minute or two.
3. Cool the bulb. Put the bulb in ice water. The valve should close.
  ✓ Check: The suction pressure falls and the superheat rises. If nothing changes in either test, the bulb charge is lost or the valve is stuck.
4. Look for ice in the orifice. If the valve works for a while and then starves, wrap it in a warm cloth. If cooling returns, there is moisture in the system: replace the filter drier and evacuate.
  ✓ Check: Cooling returns when the valve body is warmed, or this cause is ruled out.
5. Clean the strainer or replace the valve. Opening the circuit requires F-gas certification. Recover or pump down the refrigerant, open the inlet connection and clean the strainer, or replace the valve or power head that has lost its charge. Braze with nitrogen flowing, then pressure test with nitrogen, evacuate and charge by weight.
  ✓ Check: The strainer is clean or the new valve is fitted, and the system holds the pressure test and vacuum.
6. Refit the bulb and check the superheat. Refit the bulb in its original position with good contact and insulate it. Run the system stable and measure the superheat again.
  ✓ Check: The superheat is stable and within the value given for the system.
Replace a filter drier
Replace the filter drier after moisture, a compressor burnout, a restriction across the drier or whenever the circuit has been opened. Work on the refrigerant circuit requires F-gas certification; brazing is hot work.
1. Isolate or recover the refrigerant. Pump down or close valves to isolate the drier section where the system allows, otherwise recover the refrigerant with a recovery unit into a marked recovery cylinder. Break the vacuum with dry nitrogen to just above atmospheric pressure before opening. With A2L/A3 refrigerants purge with nitrogen and check with a gas detector.
  ✓ Check: The section reads slightly above 0 bar(g) on nitrogen and the pressure does not rise (no refrigerant left).
2. Choose the right drier. Use a drier of the same size or larger, with connections and capacity for the refrigerant. After a burnout or acid in the oil use a burnout (acid-removing) type and plan to replace it again after the clean-up; after moisture choose one with ample water capacity.
  ✓ Check: The new drier matches the refrigerant, size and situation, and its arrow follows the flow direction.
3. Cut out the old drier. Cut the pipes with a pipe cutter (never a hacksaw, which leaves filings) and deburr with the pipe end held downward. Keep the new drier capped until just before fitting.
  ✓ Check: Pipe ends are round, clean and burr-free.
4. Braze with nitrogen flowing. Get a fire permit, have an extinguisher at hand and remove flammable material. Purge a low flow of dry nitrogen through a regulator while brazing, and cool the drier body with a wet cloth so the desiccant is not overheated. Never use oxygen or compressed air. Flare driers are fitted with torque spanners instead.
  ✓ Check: Clean joints with full penetration, and no black oxide scale inside the pipe ends.
5. Pressure test with nitrogen. Pressurise with dry nitrogen through a regulator to the test pressure for the system, never above PS, and leak test the new joints.
  ✓ Check: The pressure holds and no leak is found at the joints.
6. Evacuate, charge and record. Evacuate with a micron gauge to the manufacturer's level (commonly below 500 micron/0.67 mbar), close off the pump and check that the vacuum holds. Charge the specified amount by weight and record the work and refrigerant amounts in the logbook.
  ✓ Check: The vacuum holds, the charge is weighed in and the logbook entry is complete.
6
The evaporator is stuck in defrost: the defrost does not terminate, or a heater contactor or hot gas valve stays on.
Remedy: Check the defrost status in the controller and the defrost termination sensor. Measure that the heaters are without voltage after defrost and that the hot gas valve closes.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check the defrost system
Incomplete defrost leaves ice on the evaporator, which reduces capacity and can block the airflow completely. Check heaters or hot gas, termination, interval, drip time and fan delay.
1. Check the defrost settings. Read defrost type, interval, maximum time, termination temperature, drip time and fan delay in the controller and compare with the manufacturer's recommendations. Check the controller clock if defrosts are scheduled.
  ✓ Check: The settings match the application and the clock is right.
2. Measure the heater resistance. Isolate, lock off and prove dead, then disconnect the heaters and measure each one. The expected resistance is R = U² ÷ P from the heater's voltage and power. Also measure insulation to earth with an insulation tester.
  ✓ Check: Each heater is close to the calculated value and has no earth fault; OL means a broken heater.
3. Measure the current during defrost. Restore power, start a manual defrost and measure the current in each heater supply with a clamp meter. Watch out for hot surfaces.
  ✓ Check: The current is about P ÷ U for each heater, or matches the nameplate defrost current.
4. Check hot gas defrost. On hot gas systems, check that the hot gas valve opens (warm pipe after the valve) and that the liquid or suction valves close as intended. Never exceed the system PS.
  ✓ Check: Hot gas reaches the evaporator and the coil warms evenly.
5. Check termination. Check that the termination sensor sits on the coldest part of the coil, usually where ice remains longest, and that the defrost ends on temperature rather than on maximum time.
  ✓ Check: The defrost ends on temperature and the coil is completely free of ice.
6. Check drip time and fan delay. After the defrost, watch the drip time and the evaporator fans when refrigeration restarts. Check that the drain and the drip tray heater work.
  ✓ Check: Fans stay off until the coil has reached the fan-delay setpoint, and water drains without refreezing in the tray.
Check a solenoid valve (liquid line or hot gas)
A solenoid valve that does not open starves the evaporator (low pressure, warm room); one that does not close lets refrigerant through when it should not (pump-down fails, flooding, or hot gas leaking into the evaporator). Check the coil first, then the valve itself.
1. Find out whether the valve should be open. Read in the controller or the wiring diagram when the valve is energised (for example with cooling demand, during pump-down or during hot gas defrost). Compare with what the system does right now.
  ✓ Check: You know whether the valve should be open or closed at this moment.
2. Check that the coil gets voltage. Measure the voltage across the coil terminals when the valve should be open. Hold a steel screwdriver blade near (not touching) the top of the coil: a live coil pulls it slightly. Check that the coil voltage on its label matches the supply.
  ✓ Check: The coil has the rated voltage (label) when the valve should be open; if not, the fault is upstream: thermostat, controller output, contactor, fuse or wiring.
3. Check the coil itself. Isolate and lock off, prove dead, and disconnect the coil. Measure its resistance and compare with the manufacturer's value; look for a burnt smell, discolouration or a cracked coil. Make sure the coil is the right type for the valve body and sits fully down on the armature tube with its nut tight.
  ✓ Check: Coil resistance matches the data (an open circuit or a very low value means a faulty coil), and the coil is the right one and correctly fitted.
4. Check that the valve opens. With the coil energised and the system running, measure the temperature on both sides of the valve with clamp thermometers. An open valve has (almost) the same liquid temperature on both sides; a clear temperature drop or frost after the valve means it is not opening fully or is restricted. Check the flow direction arrow on the valve body.
  ✓ Check: Little or no temperature difference across the valve when it should be open, and it is fitted in the direction of the arrow.
5. Check that the valve closes. De-energise the coil (or let the controller close it) and watch what happens: on a pump-down system the suction pressure must fall and the compressor must stop on the low pressure switch and stay off; a hot gas valve must leave the line after it cool during the freeze or cooling cycle.
  ✓ Check: The system pumps down and stays down, or the line after a hot gas valve stays cool; a valve that leaks through must be repaired or replaced.
6. Repair or replace. A faulty coil can be changed without opening the circuit. If the valve body leaks or sticks, opening the circuit is work for certified personnel: recover the refrigerant, replace the valve (or its internal parts) with nitrogen flowing while brazing, pressure test with dry nitrogen, evacuate and charge by weight. Protect the valve from heat while brazing as the manufacturer describes.
  ✓ Check: The valve opens and closes as it should, there is no leak at the new joints, and the work is recorded in the logbook where the refrigerant rules require it.
7
The evaporator pressure regulator (EPR) in the branch is set too high or stuck closed.
Remedy: Measure the pressure before the regulator and compare with the suction pressure and the setting. Adjust or replace the regulator.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check a pressure regulator
Evaporator, crankcase and condensing pressure regulators keep a pressure within limits. A wrong setting or a sticking valve gives wrong evaporating temperature, an overloaded compressor or low head pressure in winter.
1. Identify the regulator and its setting. Find out which type it is (evaporator pressure regulator holds the upstream pressure, crankcase pressure regulator limits the downstream pressure, condensing pressure regulator holds the upstream pressure), and the set value in the plant documentation or on the valve label.
  ✓ Check: You know which side the regulator controls and what pressure it should hold.
2. Connect gauges before and after. Connect calibrated gauges to the service ports before and after the regulator, with minimal refrigerant loss. Wear goggles and gloves.
  ✓ Check: Both gauges show stable readings that agree with the controller's transducers.
3. Compare with the setting in stable operation. Let the plant run at steady load for at least 10–15 minutes. Read the controlled pressure and the pressure drop across the valve and compare with the set value.
  ✓ Check: The controlled pressure is close to the set value; a large difference or no pressure drop at all means a wrong setting or a valve that sticks.
4. Adjust in small steps. Remove the cap and turn the adjusting spindle with the valve key a little at a time, in the direction the manufacturer gives. Wait for stable pressures after each step.
  ✓ Check: The pressure follows each adjustment smoothly and settles at the set value.
5. Check that it closes and does not hunt. Watch the gauges over a full load change (or a compressor stage switching). An evaporator pressure regulator must close when the pressure falls below its setting; a crankcase pressure regulator must throttle at start-up.
  ✓ Check: The pressure stays at the limit without creeping or swinging; the compressor current stays within the nameplate value at start-up.
6. Replace a faulty valve. If the valve does not respond or leaks through, it must be replaced. This opens the refrigerant circuit: only certified personnel (F-gas / EPA 608), recover the refrigerant, braze with nitrogen, pressure test, evacuate and record the work in the logbook.
  ✓ Check: The new valve holds the set value and the work is recorded.
Connect gauges and read pressures
Correct pressures are the basis of almost every fault diagnosis. Connect the gauges so that no refrigerant is released and the reading can be trusted.
1. Choose gauges and protection. Check the refrigerant and the system PS on the nameplate. Choose gauges and hoses rated above PS and suitable for the refrigerant; CO2 needs at least 130 bar on the high side. Use tools rated for flammable refrigerants on A2L/A3 systems. Wear goggles and gloves.
  ✓ Check: Gauges and hoses are marked for the refrigerant and a pressure above the system PS.
2. Inspect hoses and couplers. Check hose seals and that the low-loss couplers are closed. Zero the digital manifold at atmospheric pressure.
  ✓ Check: Seals are intact, couplers closed and the gauges read zero when open to air.
3. Connect to the service ports. Remove the caps and fit the couplers to the suction port and the discharge or liquid port by hand, then open them. Keep away from ignition sources on A2L/A3 systems.
  ✓ Check: The couplers are tight and a leak detector finds nothing at the connections.
4. Remove air from the hoses. Evacuate the hoses with a vacuum pump, or use short low-loss hoses that do not need purging. Never purge hoses by blowing refrigerant to the air.
  ✓ Check: No air is in the hoses and no refrigerant has been released.
5. Read and convert. Read the pressures in stable operation and note whether they are gauge or absolute. Convert to saturation temperature for the right refrigerant: dew point for suction and bubble point for liquid with zeotropic blends. On CO2 above 31 °C / 73.8 bar (the critical point) there is no saturation temperature on the high side.
  ✓ Check: You have pressures and saturation temperatures for the correct refrigerant.
6. Disconnect without loss. Close the couplers, remove them and refit the caps, which are a secondary seal. Leak test the ports.
  ✓ Check: The caps are on and no leak is detected at the ports.
8
A shut-off valve in the branch is closed, for example after service.
Remedy: Check that all valves in the branch are open.
🛠 How do I do it? Step by step
Only qualified personnel may work on refrigeration and electrical systems. Follow the manufacturer's manual and local rules; the steps are guidance, not a replacement for training.
Check valve positions and trapped liquid
A service or shut-off valve left closed or half open gives symptoms like a restriction or a low charge, and liquid shut in between closed valves can burst the pipe when it warms up. Wear goggles and gloves and never exceed the system PS.
1. Go through the circuit with the diagram. Follow the flow diagram and list every service valve, ball valve and king valve from the compressor through condenser, receiver, liquid line, evaporators and suction line.
  ✓ Check: You have a list of all valves and their required position in normal operation.
2. Check the service valves. With a valve key, check that suction, discharge and liquid service valves are fully open (back-seated). Turn slightly off the back seat only while a gauge is connected. Refit the caps afterwards.
  ✓ Check: Spindles are fully open and caps are tight.
3. Look for partly closed valves. With the system running, measure pipe temperature before and after each valve on the liquid line with a clamp thermometer. A clear temperature drop, frost or sweating after a valve means it is partly closed or restricted.
  ✓ Check: There is no noticeable temperature drop across any valve in the liquid line.
4. Check the solenoid valves. Check that solenoid valves open on a call: the coil is energised (magnet tester or voltage on the coil) and there is flow (temperature change after the valve).
  ✓ Check: Each solenoid valve opens when it is called and closes when it is not.
5. Never trap liquid. Before closing two valves on a liquid-filled section, pump the liquid out or make sure the section has a pressure relief path (relief valve or open path to the receiver). Never leave a liquid line closed at both ends.
  ✓ Check: Every section that can be closed in is empty or has a relief path.
6. CO2: check standstill protection. On R744 systems, confirm that the standstill unit, the relief valves and any pressure-holding function are in service before valves are closed, because the pressure of trapped CO2 rises quickly as it warms (see the CO2 standstill guide).
  ✓ Check: The standstill pressure stays well below PS and the relief valves are open to their outlet.

Checks from the manufacturers’ manuals (16)

Values and checks the manufacturers give for their own products; they apply to the product named.

Carel MPXone: bLo (valve blocked) or Edc (no link to the stepper driver) point to the valve side; dA1/dA2 mean the EVD ice/mini has lost a pressure or temperature probe and holds the valve closed.^[3]
RDM Mercury 3: a MOP signal from the Mercury Switch closes the liquid line solenoids for as long as the MOP alarm lasts, so cases can warm up while each case controller shows no fault.^[4]
Carel EVD evolution: The electronic valve driver never gets its start signal, so the valve stays at 0 steps: check the digital input (stand-alone) or the network link and start command from the main controller (pLAN/tLAN), and that the driver is in the right mode.^[5]
Hillphoenix Advansor CO2 booster: After start-up or rework, if a case's suction pressure transducer reads closer to liquid pressure than to suction pressure, the liquid and suction lines to that case are most likely crossed.^[6]
Hussmann transcritical CO2 racks: On a new installation the strainers in the expansion valves catch debris during the first days; Hussmann has them cleaned after at least 7 days of operation. A case that cooled at first and then weakens can simply have a clogged valve strainer.^[7]
Danfoss AK-CC55, AK-CC 550A case controllers: Pulsing valve (AKV) that does not open: measure coil voltage on the controller's valve terminals (AK-CC55: 8–9, AK-CC 550A: 5–6). If the controller calls for opening (u23 above 0 %) and voltage is on one terminal only, the relay is faulty; if voltage is right, suspect coil or blocked valve. If it does not call, check superheat reading, MOP (u26 vs n11), main switch r12 and a rack-side 'close EEV at rack shutdown'.^[8]
Danfoss KVS with AK-SM 800/AK-SC 255: Electronic evaporator pressure regulator (Danfoss KVS) in the branch: the stepper motor should measure about 52 Ω ± 10 % across red/green and across black/white; also check that the stepper module outputs are configured with the right number of steps and step rate.^[8]
ActronAir multi outdoor R32: Multi-split: the wiring of one indoor unit is landed on the terminals of another unit's pipe pair, so the wrong room gets cooling. These units can find and correct this themselves: hold the outdoor check switch for 5 s until CE shows; it takes 5–10 minutes.^[9]
Danfoss AK-PC 782B: After commissioning or a controller change, confirm that each sensor belongs to the right circuit: unplug each pressure transducer (the reading should go to ****) and spray each temperature probe with freeze spray and watch which reading falls.^[10]
Krack KR-A2L unit coolers: A unit cooler with a built-in A2L refrigerant sensor in alarm (or with the sensor faulty or unplugged) de-energises its liquid solenoid valve and runs the fans, so that evaporator stops cooling while the others work.^[11]
Toshiba multi-split RAS-5M: Multi-split: the pipes and the wiring of two rooms are crossed at the outdoor unit, so the valve for one room feeds another. Run the outdoor unit's mis-piping check.^[12]
HTPG EcoNet unit coolers: Check the electronic valve driver: the stepper type set in the controller must match the valve, and while the controller drives the valve there should be about 12 V between pairs of the coil pins (HTPG). No voltage points to the controller or cable, voltage but no movement to the valve.^[13]
HTPG EcoNet; Krack KR-A2L unit coolers: The fan delay keeps the evaporator fans off until the coil is cold (HTPG: below about 2 °C for freezers, 13 °C for coolers). In a warm room or with a faulty fan delay switch the fans stay off; Krack tests by bridging the switch.^[14]
Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: Each VRV indoor unit in cooling controls its own expansion valve on superheat calculated as gas pipe thermistor minus liquid pipe thermistor, normally 5 K (9 °F). The target rises as the room nears set point, so the valve throttles even with a large superheat. When one indoor unit cools poorly, check both thermistors against a contact thermometer before suspecting the valve or the charge.^[15]
Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): Water dripping from a J-II indoor unit that is switched off means its EEV does not close and cold refrigerant passes through. Check the indoor EEV coil: about 200 Ω ±10 % per phase at 20 °C (white–red, yellow–brown, orange–red, blue–brown), 12 V DC from the controller PCB, and that the valve clicks when power is switched on.^[16]
Fujitsu AIRSTAGE J-II VRF (outdoor units AJ*A36/45/54LALH): J-II E.14.5 (fewer indoor units than the stored maximum for 180 s) does not stop the system, so one indoor unit can stay off unnoticed. Find it with the system drawing or service tool, check its power, network line, terminal resistor and communication PCB; afterwards clear the stored unit count with function F3-41.^[17]

📝 Fill in a service report

Terms in this article

Superheat Expansion valve

Sources

Danfoss Ref Tools – Troubleshooter: solenoid valve opens partially
Danfoss Ref Tools – Troubleshooter: suction pressure too low
Carel – MPXone user manual, +0300086EN (rel. 2.0, 15.10.2019)
Resource Data Management – Mercury 3 Twin Coil case controller commissioning/user guide, PR0740/744-TWI rev. 3.3a (undated; checked 2026-10-28)
Carel – EVD evolution manual +0300005EN rel. 4.2 (24.01.2024), p. 49, table 10.a
Hillphoenix – Advansor CO2 Booster – Installation, Start-Up and Operation manual, p. A-17
Hussmann – Transcritical CO2 Rack Installation & Operation, PN 3182569_G, p. 32, 71–72
Danfoss – Troubleshooting guide for supermarket electronics BC337731384771en-000501 (2023.10), p. 3, 5
ActronAir – Multi outdoor R32 troubleshooting guide, p. 37, §6.10
Danfoss – AK-PC 782B testing and start-up guide BC479936513201en-000101 (2024.02), p. 4, §3
Krack (Hussmann) – IOM KR-A2L unit coolers, 3227892 rev. C (ES, May 2026), pp. 50-51
Toshiba – Service manual multi-split RAS-5M34G3AVG-E, SVM-23012 (2023), pp. 118-120
HTPG – Troubleshooting Guide EcoNet enabled unit coolers, HTPG0425_TSG (Jan 2025), pp. 13-14
HTPG – Troubleshooting Guide EcoNet enabled unit coolers, HTPG0425_TSG (Jan 2025) + Krack (Hussmann) – IOM KR-A2L unit coolers, 3227892 rev. C (ES, May 2026), HTPG pp. 6-7; Krack p. 46
Daikin AC (Americas) – VRV Service & Troubleshooting, Participant Guide: VRV Systems Basic Operation Guide, Engineering Data EGVUSE09-07C (09/2007), with VRV III service manual navigation and Service Checker Type III workshop slides, 3.1 Cooling Operation, p. 4; 3.4 Control of Electronic Expansion Valve, p. 7
Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 54, p. 04-62; service parts information 8, p. 04-91 (undated; copy archived 2026-11-03)
Fujitsu General – AIRSTAGE J-II Service Manual, small VRF system (R410A; outdoor units AJ*A36/45/54LALH), undated, Trouble shooting 48, p. 04-57 (undated; copy archived 2026-11-03)

One evaporator does not cool, the others do

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Trace the control and safety chain

Check a solenoid valve (liquid line or hot gas)

Check a solenoid valve (liquid line or hot gas)

Check a condenser or evaporator fan motor

Measure superheat

Check a thermostatic expansion valve

Replace a filter drier

Check the defrost system

Check a solenoid valve (liquid line or hot gas)

Check a pressure regulator

Connect gauges and read pressures

Check valve positions and trapped liquid

Checks from the manufacturers’ manuals (16)

Service report

Terms in this article

Sources