Evaporator keeps icing up

Troubleshooting · Applies to: Direct expansion refrigeration system · CO2 booster (transcritical) · Cold rooms with several evaporators

Troubleshoot an iced-up evaporator step by step: defrost, condensate drain, evaporator fan, moisture from doors and product, and low suction pressure.

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

A light frost on an evaporator below 0 °C is normal. A completely iced-up coil, however, gives too little air, low suction pressure and poor cooling, and an iced evaporator is a common cause of low suction pressure ^[1]. Defrost completely before troubleshooting further.

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

Look at how the ice lies: evenly over the whole coil points to defrost or airflow, ice in one part only to a refrigerant shortage or uneven distribution, ice at the bottom to the drain. Read the defrost log and measure the current in the heaters during a 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 defrost is not working: a faulty heater, sensor or timer, a hot-gas defrost valve that does not open, or too few and too short defrosts.
Remedy: Check the heater current, the hot-gas valve and coil where fitted, the defrost termination sensor and the settings. Increase the number or length of defrosts if needed, and check that the drip time and the fan delay after defrost are long enough.
🛠 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.
2
Electrical fault in the defrost: a failed heater, fuse, contactor or loose cable.
Remedy: Measure the resistance of each heater and the current during defrost. Check contactor, fuse and cables.
🛠 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.
Test a contactor
Worn, welded or loosely connected contactor contacts cause single-phasing, overheating and motor damage. Check the contacts, the coil and the voltage drop, and replace a worn contactor. Electrical work is for qualified personnel only.
1. Isolate and lock off. Switch off and lock the isolator, attach a tag and prove dead with a two-pole voltage tester on all line and load terminals, also the coil supply, which can come from a separate control circuit.
  ✓ Check: 0 V on all poles, the coil terminals and to earth.
2. Inspect the contacts and the mechanism. Remove the arc cover if the design allows. Look for heavily pitted, burnt or welded contacts, and discoloured terminals or melted plastic. Press the armature by hand (power off) and check that it moves freely and springs back.
  ✓ Check: Contacts are without heavy pitting or welds, and the armature moves freely.
3. Check the coil. Disconnect one coil lead and measure the coil resistance. Compare with the manufacturer's data or a new contactor of the same type.
  ✓ Check: The coil is neither open (OL) nor near 0 Ω.
4. Check the coil voltage and pull-in. Reconnect, remove your lock, restore power and use the CAT III/IV meter to measure the voltage across the coil when the controller calls for operation.
  ✓ Check: The coil voltage matches the rating printed on the coil, and the contactor pulls in firmly without humming or chattering.
5. Measure the voltage drop across each pole. With the motor running, measure from line to load terminal across each closed pole.
  ✓ Check: The drop is close to 0 V and about equal on all poles; a pole with clearly higher drop has worn contacts.
6. Tighten or replace. Isolate and lock off again. Tighten all terminals to the manufacturer's torque. Replace a worn contactor with one of the same coil voltage and at least the same motor rating (AC-3), then restart and measure the current on each phase.
  ✓ Check: Terminals are tight, and the motor current is about equal on all phases after restart.
3
The sensor that ends the defrost is badly placed or reads wrong, so the defrost stops before the coil is free of ice.
Remedy: Check the sensor location and compare it with a calibrated thermometer.
🛠 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 sensor against a reference
A temperature or pressure sensor that reads wrong makes the controller regulate wrong: wrong superheat, wrong defrost, false alarms. Compare it with a calibrated reference before changing settings or parts.
1. Fit the reference next to the sensor. Fasten a calibrated thermometer probe beside the temperature sensor on the same pipe or in the same air stream, with heat-conducting paste and insulation. For a pressure sensor, connect a calibrated gauge to a service port at the same point in the circuit. Wear goggles and gloves when connecting to the refrigerant circuit.
  ✓ Check: The reference reading is steady for a few minutes.
2. Compare with the controller reading. Read the live value of the sensor in the controller display or service tool at the same time as the reference.
  ✓ Check: The difference is within the tolerance given by the sensor or controller manufacturer (for a temperature sensor typically within about 1 K).
3. Check offsets and sensor type. In the controller parameter list, check that the sensor type (e.g. NTC, PT1000, 0–10 V, 4–20 mA), the pressure range and any calibration offset match the fitted sensor. Record the original values before changing anything.
  ✓ Check: Sensor type, range and offset in the controller match the sensor data sheet.
4. Measure the sensor resistance. Isolate and lock off the controller, disconnect the sensor and measure its resistance with a multimeter while the reference shows the temperature at the sensor. Compare with the manufacturer's resistance table. A PT1000 reads 1000 Ω at 0 °C; an ice-water bath gives a simple 0 °C check point.
  ✓ Check: The resistance matches the table value for the reference temperature; OL or 0 Ω means a broken or shorted sensor or cable.
5. Check the sensor location and mounting. Inspect the pipe clamp or pocket: firm metal-to-metal contact, heat-conducting paste, insulation against the surrounding air and the correct position (e.g. evaporator outlet, not in the air stream of a fan).
  ✓ Check: The sensor sits firmly at the correct point and is insulated.
6. Reconnect and confirm. Reconnect the sensor, restore power and compare the reading with the reference once more. Replace a sensor that is out of tolerance with one of the same type.
  ✓ Check: Controller and reference agree within the tolerance.
4
Flowchart C
The fan is stopped or running too slowly, so the airflow is too low.
Remedy: Check the fan motor, run capacitor, motor protection and controls.
🛠 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).
Test the start/run capacitor and the start relay
Faulty capacitors and start relays are common reasons why a compressor or fan motor hums, overheats or will not start.
1. Isolate and lock off. Switch off and lock the isolator (or unplug the unit) and remove the cover of the compressor terminal box. Set the multimeter to AC volts and measure between phase and neutral, and between each phase and earth.
  ✓ Check: The meter shows 0 V AC on the incoming terminals.
2. Photograph, discharge and disconnect the capacitor. Photograph the wiring with your phone so that you can put it back the same way. Discharge the capacitor through a resistor of about 20 kΩ, 5 W with insulated leads, held across the terminals for a few seconds. Never short it with a screwdriver: the spark damages the terminals and the capacitor. Then pull off the cables.
  ✓ Check: The meter shows 0 V DC directly across the capacitor terminals.
3. Measure the capacitance. Set the multimeter to capacitance (µF) and measure across the terminals. Compare with the rating printed on the capacitor. Tecumseh gives the tolerances: a start capacitor −0/+20 % of its rating, a run capacitor ±10 %. A swollen or leaking capacitor, or one that reads 0 µF, is faulty.
  ✓ Check: The measured value is within the tolerance of the printed rating.
4. Check the start relay. Pull the relay off the compressor pins and look for burn marks or a smell of burnt plastic. A current relay is tested upright: L–S open, L–M closed (Tecumseh). A PTC starter must cool for about 3 minutes before a resistance reading means anything; compare it with the value in the compressor or starter data sheet (usually a few to some tens of ohms at room temperature).
  ✓ Check: The relay contacts are as described, or the PTC reads its data-sheet value; OL (open) means the part is faulty.
5. Replace the faulty part and test run. Fit a new part with exactly the same data: capacitance in µF, voltage rating and relay type. Mount the relay the right way up. Reconnect from your photo, refit the cover, switch on and start the unit. If capacitors or relays fail again, look for the cause: low supply voltage, short cycling or a relay of the wrong type.
  ✓ Check: The compressor starts smoothly, without humming and without the motor protection tripping.
Check motor protection setting and trips
Motor protection stops the compressor or fan before the motor burns. A wrong setting causes needless trips or no protection at all, and a protection that is reset again and again without finding the cause destroys the motor.
1. Read the trip and the alarm log. Note which protection has tripped (overload relay, motor protection breaker, electronic module or inverter) and read the alarm log in the controller for time and number of trips.
  ✓ Check: You know which device tripped, when and how often.
2. Check the setting against the nameplate. Compare the current setting of the overload relay or motor protection breaker with the value the compressor or motor manufacturer gives (for example the maximum continuous current on the nameplate or in the data sheet). Never set it higher than that value.
  ✓ Check: The setting agrees with the manufacturer's value.
3. Read the electronic protection module. On compressors with a thermistor protection module (for example Kriwan INT69), read any diagnosis LED or flash code with the module's manual. Check that its supply voltage is correct.
  ✓ Check: The module shows normal, or the code is identified in the manual.
4. Measure current in operation. With the protection reset and the cause not obviously dangerous, start the unit and measure the current on every phase with a clamp meter (CAT III/IV, insulated, no loose clothing). Compare with the nameplate and check that the phases are balanced.
  ✓ Check: The current is below the protection setting and nearly equal on all phases.
5. Count the starts. Count starts per hour from the controller log or by watching. Compare with the maximum starts per hour in the compressor data. Too many starts point to short cycling (setpoint, differential or low pressure switch).
  ✓ Check: The number of starts is within the compressor manufacturer's limit.
6. Find the cause before resetting again. Look for the cause of the trip: high condensing pressure, low or unbalanced voltage, a lost phase, a mechanical fault or a winding fault. Isolate and lock off before opening terminals. Reset only when the cause is found and fixed.
  ✓ Check: The cause is found and corrected, and the unit runs without new trips.
5
Flowchart B
The meltwater freezes in the drip tray or drain line and builds up ice from below.
Remedy: Thaw and clean the drain. Check the drain line heater and that the pipe has a fall.
🛠 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.
Clear the condensate drain
A blocked or frozen drain lets melt water overflow into the room and freeze on the floor or the coil. Clear it, then fix the cause: fall, trap or a dead heater cable.
1. Isolate and protect yourself. Isolate, lock off and prove dead the evaporator before working at the tray. Wet cold-room floors are slippery: wear shoes with grip and tell someone you are inside.
  ✓ Check: The meter shows 0 V on the evaporator terminals.
2. Thaw and clean the tray and drain. Thaw ice in the tray and drain with warm water. Clean out slime and debris with a flexible drain brush.
  ✓ Check: Warm water poured into the tray runs straight out without backing up.
3. Measure the heater cable resistance. With the power still off, disconnect the drain heater cable and measure its resistance. Calculate the expected value from the rating (R = U² / P) or take it from the data sheet. Check the fuse and the thermostat, if any.
  ✓ Check: The resistance is close to the expected value; OL (open) means a broken cable.
4. Check the heater in operation. Reconnect, refit covers and switch on. Measure the current with a clamp meter and feel or measure that the drain warms along its full length inside the cold room.
  ✓ Check: The current matches the rating and the drain is warm all the way out of the room.
5. Check fall and trap. Check with a spirit level that the drain has a continuous fall with no sags. In freezer rooms the trap should be outside the cold room, in the warm area, so it cannot freeze.
  ✓ Check: The drain falls all the way, the trap holds water and is placed where it cannot freeze.
6. Run a defrost and check. Start a defrost and watch the melt water.
  ✓ Check: All melt water leaves the room and nothing refreezes in the tray or drain.
Test drain, tray and frame heaters
Heater cables keep drains, drip trays, door frames and pressure relief ports free of ice. A broken heater gives a blocked drain, water on the floor or a frozen door.
1. Isolate, lock off and prove dead. Switch off and lock the isolator for the heater circuit, attach a tag and prove dead with a two-pole voltage tester (checked on a known live source before and after) at the heater terminals.
  ✓ Check: The tester shows no voltage between the conductors and between each conductor and earth.
2. Check fuse, thermostat and control output. Check the fuse or circuit breaker for the heater and the thermostat or controller relay that switches it. A drain heater is often switched only during defrost; a frame heater is often on all the time. Read the wiring diagram.
  ✓ Check: The fuse is intact and you know when the heater should be on.
3. Measure the resistance. Disconnect the heater leads and measure resistance across the element. Calculate the expected value from the nameplate: R = V² / P (for example 230 V, 100 W gives about 530 Ω). Self-regulating cables cannot be judged this way; check them by current instead.
  ✓ Check: The reading is close to the calculated value; OL (open) means a broken element, near 0 Ω means a short.
4. Measure insulation resistance to earth. Use an insulation tester at 500 V DC between the joined heater conductors and the earth conductor or metal braid, with the heater disconnected from any electronics.
  ✓ Check: At least 1 MΩ (IEC 60364-6 minimum for 230/400 V circuits); a low or falling value means moisture in the cable or end seal.
5. Measure voltage and current in operation. Reconnect, refit the covers and switch on. Make the controller call for the heater (or start a defrost), then measure voltage at the heater and current with a clamp meter. Expected current is about P / V.
  ✓ Check: The heater gets full voltage and draws roughly the nameplate current.
6. Check the heater position. Check that the drain heater runs the full length of the drain inside the cold space and through the wall, that the tray heater lies against the tray, and that the frame heater warms the whole door frame.
  ✓ Check: After the next defrost the drain runs freely and the frame is warm and free of ice.
6
Flowchart D
High moisture means ice forms faster than the defrost can melt it.
Remedy: Repair seals and door closers, fit a strip curtain at the door and adjust the defrost.
🛠 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 doors, seals and the cold-room envelope
Warm, moist air that leaks into a cold room gives ice on the evaporator, frost on the walls and a higher load. Doors and penetrations are the usual weak points.
1. Test the inside release first. Before you work inside, check from inside that the door opens with the emergency release, even when it is locked from outside. Test the person-trapped alarm button if there is one.
  ✓ Check: The door opens from inside at once, and the alarm sounds where it should.
2. Inspect the gaskets. Look for tears, flattened or hard sections and ice on the gaskets. Close the door on a strip of paper at several points around the frame and pull it out.
  ✓ Check: The paper is held firmly all the way around; places where it slides out easily need a new or adjusted gasket.
3. Check closing and door alarm. Open the door and let it go: check that the closer or self-closing hinges shut and latch it. Leave it open and check that the door-open alarm sounds after its set delay.
  ✓ Check: The door closes and latches by itself, and the alarm works.
4. Check frame and relief port heating. Feel the door frame and, on freezer rooms, the pressure relief port: they should be warm and free of ice and condensation. If not, test the heater circuit (isolate, lock off and prove dead before you touch its terminals; see the guide on heater cables).
  ✓ Check: Frame and relief port are warm and dry, and the relief port flap moves freely.
5. Check strip curtains. Check that the strips overlap, hang down to the floor and are not missing, torn or tied aside.
  ✓ Check: The curtain closes the whole opening.
6. Inspect panel joints and penetrations. Look for frost or ice on the inside and damp on the outside at panel joints and pipe or cable penetrations; an IR camera makes this quicker. Check that penetrations are sealed and that the vapour barrier on the warm side is intact.
  ✓ Check: No frost spots or damp patches, and every penetration is sealed.
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.
7
Flowchart E
Low suction pressure gives a very low evaporating temperature and rapid ice formation.
Remedy: Follow the low-pressure troubleshooting.
Continue with: System trips on low pressure →

Checks from the manufacturers’ manuals (18)

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

Danfoss AK-CC 550/AK-CC55: adaptive defrost raises its own alarm (A75 or X02) when ice blocks the air flow through the coil, so check the alarm list before changing defrost times by hand.^[2]
Danfoss AK-RC 251: E2 at defrost start, when evaporator probe S2 reads about 22 K (40 °F) above room probe S1, points to moisture in the S2 probe; defrost then ends on time instead.^[3]
On Full Gauge TC-940R and TC-900E controllers a flashing dot at the lower right of the display means the last defrost stopped on the time limit, not on the evaporator temperature; check for burnt-out heaters, hot gas not flowing, a stopped fan, too long an interval between defrosts or too short a maximum defrost time.^[4]
Heatcraft unit coolers: Electric defrost heaters that have crept away from the coil lose contact, so the coil does not clear evenly; compare heater current with the data plate, check even contact, lower the defrost termination temperature and watch the frost pattern.^[5]
Heatcraft unit coolers: Uneven frosting of the coil: a wrong or missing distributor nozzle, an evaporator placed too close to the door, or a defrost termination temperature set too low.^[6]
Temperzone air-to-air units, UC8: Air conditioning indoor coil: frost forms when the air through the coil is too little or too cold: blocked filter, closed supply dampers, too much cold fresh air, too low fan speed or running cooling with a very low return air temperature.^[7]
Danfoss AK-SM 800/800A, AK-SC 255: Manual defrost that stops at once: if the termination sensor is already above the termination temperature, or a termination input already shows on, the defrost ends immediately. With defrost schedules enabled, a 'max concurrent defrosts' of 0 blocks it too.^[8]
Dixell XR/XW/XM controllers; Carel ir33: Place the defrost termination probe in the fins where most ice forms (the coldest part, among the fans), away from heater elements, with the bulb pointing up so water cannot get in. Keep probe cables away from contactors and power cables; Carel allows up to about 10 m of probe cable.^[9]
Temperzone ducted units with UC8 controller: In ducted air conditioning, closed zone dampers (or too few zones open) cut the airflow so the indoor coil frosts; check the dampers, damper control and indoor fan speed as well as the filter.^[10]
Krack MS-A2L unit coolers: Unit cooler fans stay off after defrost because the coil never gets cold enough to reset the fan-delay thermostat (room too warm, e.g. at pull-down), while the coil keeps cooling and ices up. Jumper the fan-delay switch (terminals F and B) to test it.^[11]
HTPG EcoNet unit coolers: Start a manual defrost, check with a clamp meter that every heater draws current, and time the defrost. HTPG unit coolers normally need about 20-30 minutes; a termination sensor that reads too high ends the defrost early, so compare it with the resistance table.^[12]
HTPG EcoNet center-mount and low-profile unit coolers: On evaporators with two heater groups, each termination sensor must sit on the coil whose heaters it controls. Swapped sensors, or a controller set for one coil instead of two, leave one side without defrost.^[12]
Hoshizaki Steelheart reach-in freezers: The defrost limit thermostat (bimetal) on a freezer coil is open or faulty, so the heaters never get power. On Hoshizaki reach-ins it opens at about 49 °C and closes at about 21 °C; it must be closed when the coil is cold.^[13]
Hussmann FS-OB display cases: Display case: missing coil close-off panels or a return air grille blocked by debris or product lets air bypass the coil, and the coil freezes over.^[14]
True reach-ins; Hussmann glass door cases: Cabinet doors do not seal: the cabinet is not level, the self-closer is weak or the gaskets have gaps after transport. True expects self-closing doors to close from 50-75 mm open; Hussmann conditions new gaskets by running the anti-sweat heaters 2-4 hours before start.^[15]
Heatcraft MAC / unit coolers: Freezer drain lines: the drain line heater must be on continuously, about 20 W per foot (about 65 W/m) for −18 °C rooms and 30 W per foot (about 100 W/m) for −29 °C rooms; the line is insulated, falls at least 2 cm per metre and its trap sits in a warm space, never inside the freezer.^[16]
Hitachi RUA-NP4ATS…RUA-NP10ATS self-contained air conditioners (R410A): The RUA-NP evaporator fan is belt driven with an adjustable motor pulley: one turn of the movable flange changes the fan speed by about 40–49 rpm depending on the model (toward the fixed flange is faster). After any adjustment set the belt to about 20 mm deflection under one finger. Too low a fan speed or a slipping belt lowers the air flow below 80 % of nominal and can ice the coil.^[17]
Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: VRV indoor freeze-up prevention starts when the liquid pipe thermistor (R2T) has been at −1 °C (30.2 °F) or lower for a total of 40 minutes, or at −5 °C (23 °F) or lower for a total of 10 minutes; for two- and three-way cassettes the limits are 1 °C for 15 minutes or 0 °C for 1 minute continuously. The expansion valve then closes, the drain pump runs and the fan stays at low speed until the coil has been at 7 °C (44.6 °F) or more for 10 minutes. A unit that often goes into this mode needs a check of air flow, filter and R2T.^[18]

📝 Fill in a service report

Terms in this article

Superheat

Sources

Danfoss Ref Tools – Troubleshooter: suction pressure too low
Danfoss – User guide AK-CC55 Single Coil SW 2.4x, BC364229688105en-010401 (2026.08); User guide AK-CC55 Compact SW 2.3x, BC365212986197en-000302 (2026.08)
Danfoss – User guide temperature controller for walk-in coolers and freezers AK-RC 251, BC364433930186en-000101 (2021.02)
Full Gauge Controls – TC-940R controller for refrigeration and defrost with alarm output (Version 03), 5.7 Indicators and alarms
Heatcraft – MAC condensing units Installation & Operation Manual, H-IM-67H (March 2018), p. 40 (preventive maintenance) and p. 42 (E7, LPS)
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Krack – MS-A2L Series unit cooler I&O manual 3237208, p. 48, Troubleshooting
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Hitachi – Self-Contained Air Conditioners RUA-NP4ATS…RUA-NP10ATS (R410A), Installation, Operation and Maintenance Instructions Manual 65MP3197-A2 (Bosch Home Comfort Philippines); undated, PDF file created 2025, 4. Fan Speed Adjustment, Table 1-1/1-2, p. 8–9
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, 4.3 Indoor Coil Freeze Prevention Control, p. 13

Evaporator keeps icing up

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Check the defrost system

Check the defrost system

Test a contactor

Check a sensor against a reference

Check a condenser or evaporator fan motor

Test the start/run capacitor and the start relay

Check motor protection setting and trips

Clear the condensate drain

Test drain, tray and frame heaters

Check doors, seals and the cold-room envelope

Check the defrost system

Checks from the manufacturers’ manuals (18)

Service report

Terms in this article

Sources