Low oil level or oil pressure alarm

Troubleshooting · Applies to: Direct expansion refrigeration system · CO2 booster (transcritical) · Ammonia with pump circulation (R717) · Cold rooms with several evaporators

Low oil level or oil pressure alarm: oil leaks, poor oil return, refrigerant in the oil, oil equalisation and the oil pump.

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

Oil leaving the compressor has to come back with the suction gas. If the level drops, the oil is either held up in the system, in evaporators and pipes with too low a gas velocity, or it has leaked out. Danfoss mentions, among other things, a missing oil separator and too little fall on horizontal suction lines ^[1]. Read the oil level with the compressor running after a period of stable operation.

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

Read the oil level in the compressor sight glass after at least 30 minutes of stable operation and compare with the manufacturer's minimum and maximum. For an oil pressure alarm: measure the net oil pressure (oil pressure minus crankcase pressure) and compare with the manufacturer's limit. If the level is correct but the net oil pressure is low, the fault is in the oil pump, oil filter or oil pressure switch (the last cause), not a lack of oil.

📏 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 B
The oil is held up in the system: too low gas velocity in risers, suction lines without fall or too short run times.
Remedy: Check riser sizes and oil traps, the fall of the suction line towards the compressor and the minimum run time. Consider an oil separator.
🛠 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 oil return and the oil separator
Oil that leaves the compressor must come back. Poor oil return gives low oil level, oil pressure trips and worn compressors, often only at low load.
1. Read the compressor oil level. With the compressor running in stable operation, read the oil level in the sight glass and compare with the level the compressor manufacturer gives (often between a quarter and three quarters of the glass). Note any foaming.
  ✓ Check: The level is within the manufacturer's range and stays steady over several minutes.
2. Check the oil separator return. Measure the temperature of the oil return line from the separator with a clamp thermometer, before and after the float or solenoid. Be careful: discharge-side parts can be over 100 °C.
  ✓ Check: The return line warms up periodically as the float opens; a line that is always cold means a blocked return, one that is always hot means a float that sticks open and returns hot gas.
3. Check filters, solenoids and level regulators. On racks with an oil management system, check the oil reservoir level, the return filter and the solenoid, and that each compressor's level regulator fills and stops. Check the equalising lines between compressors.
  ✓ Check: Every compressor keeps its oil level and the reservoir does not empty or overfill.
4. Check riser sizes against the sizing table. Compare the suction and discharge riser sizes with a pipe sizing table for the lowest capacity the system runs at. Too large a riser does not carry the oil up at low load.
  ✓ Check: The gas velocity in the risers at minimum load is enough to carry oil according to the table or the sizing tool.
5. Check oil traps and the fall of the suction line. Check that long suction risers have an oil trap at the bottom (and intermediate traps on tall risers as the design guidelines require) and that horizontal suction lines have a continuous fall towards the compressor with no sags.
  ✓ Check: The traps are in place and no section of horizontal suction line slopes away from the compressor.
6. Run at low load and check again. Run the plant at its lowest normal capacity for a while and read the oil levels again. Correct the faults found, and add only the oil type the compressor manufacturer specifies.
  ✓ Check: The oil level stays within the range also at low load.
2
Flowchart A
Leak: oil and refrigerant leak out together.
Remedy: Leak test, repair the leak and add the correct oil type and quantity according to the manufacturer.
🛠 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.
Leak test a refrigeration system
Find leaks systematically, confirm the repair and record the work. Work that opens the refrigerant circuit requires F-gas certification (Regulation (EU) 2024/573 or national equivalent, EPA 608 in the USA).
1. Prepare the system and the detector. The system must be under pressure (running, or at standstill with warm ambient). Stop fans in the area to get calm air and ventilate low or enclosed spaces. With A2L/A3 refrigerants use no ignition sources and only detectors rated for them; with CO2 (R744) remember the very high pressure. Check the electronic detector (EN 14624) against its reference leak or as the manufacturer describes.
  ✓ Check: The detector reacts to the reference leak, and the air around the system is calm.
2. Search with the electronic detector. Move the probe slowly, a few centimetres per second, close to every brazed joint, flare, valve stem, Schrader core, gauge and coil bend. Refrigerants are heavier than air, so hold the probe at and below each joint. Follow the circuit in a fixed order so that nothing is missed.
  ✓ Check: Every joint along the circuit has been passed, and each alarm has been repeated at the same spot.
3. Confirm with bubble solution. Spray bubble leak solution on each suspected point and watch it for at least a minute; small leaks form bubbles or foam slowly.
  ✓ Check: Bubbles or foam pinpoint the leak, or the point is confirmed tight.
4. Use UV dye only where it is approved. If the system already contains a UV dye approved by the compressor manufacturer, light the joints with a UV lamp and wear UV glasses.
  ✓ Check: A fluorescent trace marks the leak, or no trace is visible.
5. Repair correctly. Recover the refrigerant (or isolate the section), repair with nitrogen flowing when brazing, pressure test with nitrogen, evacuate and charge by weight. Never top up refrigerant 'to test' without finding and repairing the leak.
  ✓ Check: The repaired joint holds the nitrogen test pressure and the system is charged with the specified amount.
6. Record and re-check. Write the leak location, cause, repair, refrigerant amount and your certificate number in the equipment logbook. Check the repair again within one month, as the F-gas regulation requires.
  ✓ Check: The logbook entry is complete and a follow-up check is booked.
Check oil level and oil pressure
Too little oil, or too little oil pressure, wears out a compressor quickly. Check the level in stable operation, measure the net oil pressure on compressors with an oil pump, and find out where the oil went before topping up.
1. Read the oil level in stable operation. Let the compressor run stable for at least 15 minutes and read the level in the sight glass. Compare with the marks or the range in the compressor manual. Foaming oil points to refrigerant in the oil.
  ✓ Check: The level is within the range in the manual and the oil is not foaming.
2. Measure the net oil pressure. On compressors with an oil pump, connect a gauge to the oil pump outlet and one to the crankcase or suction side. Net oil pressure = oil pump pressure − crankcase pressure. Wear goggles and gloves; the pipes can be hot.
  ✓ Check: The net oil pressure agrees with the compressor manufacturer's value.
3. Check the oil pressure switch. Check that the oil differential pressure switch is set as the compressor manual says and that its time delay is correct. If the procedure in the manual allows, test that it stops the compressor when the net oil pressure is too low.
  ✓ Check: The switch is set as in the manual and stops the compressor after its time delay.
4. Find the cause of a low level. Before adding oil, look for the cause: oil trapped in the evaporator or suction line, a faulty oil separator return or level regulator, a leak, or frequent short cycles.
  ✓ Check: The cause of the low level is found, or the level was only low after a recent repair.
5. Top up with the right oil. Use only the oil type on the compressor nameplate (POE, PAG, mineral or PAO) from a sealed container; POE and PAG absorb moisture. Pump it in with an oil pump through the service valve so no air enters, in small amounts.
  ✓ Check: The level is back in the correct range after stable operation, and not above it.
6. Record. Record the oil type and quantity added and the measured oil pressure in the logbook.
  ✓ Check: The logbook shows what was done.
Leak check of refrigeration systems: F-gas intervals and steps →
3
Flowchart C
Refrigerant has migrated to the crankcase during standstill and dilutes the oil, which foams out at start.
Remedy: Repair or connect the crankcase heater. Consider a pump-down before stopping.
🛠 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.
Test the crankcase heater
The crankcase heater keeps refrigerant from condensing in the compressor oil at standstill. A broken or loose heater leads to foaming oil and liquid slugging at start.
1. Isolate, lock off and prove dead. Switch off and lock the isolator and prove dead at the heater terminals. The heater and the compressor shell can be hot.
  ✓ Check: The voltage tester shows 0 V on the heater terminals.
2. Measure the heater resistance. Disconnect one heater lead and measure the resistance. Work out the expected value from the rating: R = V² / P (for example 230 V and 50 W gives about 1060 Ω). Self-regulating (PTC) heaters show a different value; compare with their data sheet.
  ✓ Check: The resistance is close to the calculated or data-sheet value; OL means the heater is broken.
3. Check the insulation to earth. Measure between the heater leads and the compressor body. A heater with a damaged sheath often leaks to earth and trips the earth-fault breaker.
  ✓ Check: The meter shows no connection (OL) between the leads and earth.
4. Check the position. Check that the heater sits where the compressor manual says, usually tight around the lower shell near the oil sump, with full contact and the clamp tightened.
  ✓ Check: The heater has full contact with the shell at the position in the manual.
5. Check that it is on at standstill. Reconnect, restore power and keep the compressor stopped. Measure the voltage at the heater and its current (I = P / V). After a while, check the shell temperature with a contact thermometer.
  ✓ Check: The heater has voltage and draws about its rated current at standstill, and the shell bottom is warmer than the surroundings.
6. Check the control. On units where the heater switches off while the compressor runs, check that it switches on again when the compressor stops.
  ✓ Check: The heater is on whenever the compressor is stopped.
Check the pump-down function
In pump-down the liquid solenoid valve closes and the compressor pumps the low side down until the low pressure switch stops it. If the valve leaks or the switch is set wrong, the compressor short cycles or liquid migrates to the crankcase.
1. Connect gauges. Connect gauges to the suction and discharge service ports with short hoses and minimal refrigerant loss. Note the low pressure switch cut-out and cut-in settings from the switch or the controller.
  ✓ Check: Both gauges show stable operating pressures and the switch settings are noted.
2. Start the pump-down. Make the thermostat or controller stop the cooling call (raise the setpoint or open the thermostat contact) so the liquid solenoid valve is de-energised.
  ✓ Check: The coil voltage on the solenoid valve drops to 0 V (measured with a CAT III/IV meter) and the valve clicks closed.
3. Watch the suction pressure fall. Watch the suction gauge while the compressor keeps running and pumps refrigerant into the condenser and receiver.
  ✓ Check: The suction pressure falls steadily towards the cut-out setting; it does not stop falling well above it.
4. Check the low pressure cut-out. Note the pressure at which the compressor stops and compare it with the setting.
  ✓ Check: The compressor stops at the set cut-out pressure, within the accuracy of your gauge.
5. Check that the solenoid valve holds. With the compressor stopped, watch the suction pressure for several minutes and feel or measure the pipe temperature after the solenoid valve. A slow rise as the evaporator warms up is normal; a fast rise, or a cold pipe or frost after the valve, means the valve is passing liquid.
  ✓ Check: The suction pressure rises only slowly and the pipe after the valve is not getting cold.
6. Check the restart. Restore the cooling call. The solenoid valve should open, the suction pressure rise above the cut-in and the compressor start. Without a cooling call the compressor must not restart on its own within a short time; check the anti-short-cycle timer in the controller.
  ✓ Check: The compressor restarts only when cooling is called for and does not cycle on the low pressure switch alone.
4
The oil separator does not return the oil: the float valve, return filter or solenoid in the return line is blocked or stuck closed.
Remedy: Feel the oil return line from the separator: it should get warm at intervals while running. Clean or replace the float valve, filter or solenoid.
🛠 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 oil return and the oil separator
Oil that leaves the compressor must come back. Poor oil return gives low oil level, oil pressure trips and worn compressors, often only at low load.
1. Read the compressor oil level. With the compressor running in stable operation, read the oil level in the sight glass and compare with the level the compressor manufacturer gives (often between a quarter and three quarters of the glass). Note any foaming.
  ✓ Check: The level is within the manufacturer's range and stays steady over several minutes.
2. Check the oil separator return. Measure the temperature of the oil return line from the separator with a clamp thermometer, before and after the float or solenoid. Be careful: discharge-side parts can be over 100 °C.
  ✓ Check: The return line warms up periodically as the float opens; a line that is always cold means a blocked return, one that is always hot means a float that sticks open and returns hot gas.
3. Check filters, solenoids and level regulators. On racks with an oil management system, check the oil reservoir level, the return filter and the solenoid, and that each compressor's level regulator fills and stops. Check the equalising lines between compressors.
  ✓ Check: Every compressor keeps its oil level and the reservoir does not empty or overfill.
4. Check riser sizes against the sizing table. Compare the suction and discharge riser sizes with a pipe sizing table for the lowest capacity the system runs at. Too large a riser does not carry the oil up at low load.
  ✓ Check: The gas velocity in the risers at minimum load is enough to carry oil according to the table or the sizing tool.
5. Check oil traps and the fall of the suction line. Check that long suction risers have an oil trap at the bottom (and intermediate traps on tall risers as the design guidelines require) and that horizontal suction lines have a continuous fall towards the compressor with no sags.
  ✓ Check: The traps are in place and no section of horizontal suction line slopes away from the compressor.
6. Run at low load and check again. Run the plant at its lowest normal capacity for a while and read the oil levels again. Correct the faults found, and add only the oil type the compressor manufacturer specifies.
  ✓ Check: The oil level stays within the range also at low load.
5
Flowchart D
Oil equalisation between the compressors does not work: oil level regulator, equalising line or check valve.
Remedy: Check the oil level regulators, the oil separator return and its filter. Make sure the equalising lines are routed correctly.
🛠 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 oil return and the oil separator
Oil that leaves the compressor must come back. Poor oil return gives low oil level, oil pressure trips and worn compressors, often only at low load.
1. Read the compressor oil level. With the compressor running in stable operation, read the oil level in the sight glass and compare with the level the compressor manufacturer gives (often between a quarter and three quarters of the glass). Note any foaming.
  ✓ Check: The level is within the manufacturer's range and stays steady over several minutes.
2. Check the oil separator return. Measure the temperature of the oil return line from the separator with a clamp thermometer, before and after the float or solenoid. Be careful: discharge-side parts can be over 100 °C.
  ✓ Check: The return line warms up periodically as the float opens; a line that is always cold means a blocked return, one that is always hot means a float that sticks open and returns hot gas.
3. Check filters, solenoids and level regulators. On racks with an oil management system, check the oil reservoir level, the return filter and the solenoid, and that each compressor's level regulator fills and stops. Check the equalising lines between compressors.
  ✓ Check: Every compressor keeps its oil level and the reservoir does not empty or overfill.
4. Check riser sizes against the sizing table. Compare the suction and discharge riser sizes with a pipe sizing table for the lowest capacity the system runs at. Too large a riser does not carry the oil up at low load.
  ✓ Check: The gas velocity in the risers at minimum load is enough to carry oil according to the table or the sizing tool.
5. Check oil traps and the fall of the suction line. Check that long suction risers have an oil trap at the bottom (and intermediate traps on tall risers as the design guidelines require) and that horizontal suction lines have a continuous fall towards the compressor with no sags.
  ✓ Check: The traps are in place and no section of horizontal suction line slopes away from the compressor.
6. Run at low load and check again. Run the plant at its lowest normal capacity for a while and read the oil levels again. Correct the faults found, and add only the oil type the compressor manufacturer specifies.
  ✓ Check: The oil level stays within the range also at low load.
6
Flowchart E
The oil pressure switch, the oil pump or a clogged oil filter gives an alarm although the level is correct.
Remedy: Measure the oil pressure with a gauge and compare with the manufacturer's value. Replace the filter or a faulty switch; low net oil pressure with a correct level may mean a worn pump.
🛠 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 oil level and oil pressure
Too little oil, or too little oil pressure, wears out a compressor quickly. Check the level in stable operation, measure the net oil pressure on compressors with an oil pump, and find out where the oil went before topping up.
1. Read the oil level in stable operation. Let the compressor run stable for at least 15 minutes and read the level in the sight glass. Compare with the marks or the range in the compressor manual. Foaming oil points to refrigerant in the oil.
  ✓ Check: The level is within the range in the manual and the oil is not foaming.
2. Measure the net oil pressure. On compressors with an oil pump, connect a gauge to the oil pump outlet and one to the crankcase or suction side. Net oil pressure = oil pump pressure − crankcase pressure. Wear goggles and gloves; the pipes can be hot.
  ✓ Check: The net oil pressure agrees with the compressor manufacturer's value.
3. Check the oil pressure switch. Check that the oil differential pressure switch is set as the compressor manual says and that its time delay is correct. If the procedure in the manual allows, test that it stops the compressor when the net oil pressure is too low.
  ✓ Check: The switch is set as in the manual and stops the compressor after its time delay.
4. Find the cause of a low level. Before adding oil, look for the cause: oil trapped in the evaporator or suction line, a faulty oil separator return or level regulator, a leak, or frequent short cycles.
  ✓ Check: The cause of the low level is found, or the level was only low after a recent repair.
5. Top up with the right oil. Use only the oil type on the compressor nameplate (POE, PAG, mineral or PAO) from a sealed container; POE and PAG absorb moisture. Pump it in with an oil pump through the service valve so no air enters, in small amounts.
  ✓ Check: The level is back in the correct range after stable operation, and not above it.
6. Record. Record the oil type and quantity added and the measured oil pressure in the logbook.
  ✓ Check: The logbook shows what was done.
7
The oil pressure switch cable is loose, or its time delay is wrong.
Remedy: Check cables and terminals, and the switch delay and reset.
🛠 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 oil level and oil pressure
Too little oil, or too little oil pressure, wears out a compressor quickly. Check the level in stable operation, measure the net oil pressure on compressors with an oil pump, and find out where the oil went before topping up.
1. Read the oil level in stable operation. Let the compressor run stable for at least 15 minutes and read the level in the sight glass. Compare with the marks or the range in the compressor manual. Foaming oil points to refrigerant in the oil.
  ✓ Check: The level is within the range in the manual and the oil is not foaming.
2. Measure the net oil pressure. On compressors with an oil pump, connect a gauge to the oil pump outlet and one to the crankcase or suction side. Net oil pressure = oil pump pressure − crankcase pressure. Wear goggles and gloves; the pipes can be hot.
  ✓ Check: The net oil pressure agrees with the compressor manufacturer's value.
3. Check the oil pressure switch. Check that the oil differential pressure switch is set as the compressor manual says and that its time delay is correct. If the procedure in the manual allows, test that it stops the compressor when the net oil pressure is too low.
  ✓ Check: The switch is set as in the manual and stops the compressor after its time delay.
4. Find the cause of a low level. Before adding oil, look for the cause: oil trapped in the evaporator or suction line, a faulty oil separator return or level regulator, a leak, or frequent short cycles.
  ✓ Check: The cause of the low level is found, or the level was only low after a recent repair.
5. Top up with the right oil. Use only the oil type on the compressor nameplate (POE, PAG, mineral or PAO) from a sealed container; POE and PAG absorb moisture. Pump it in with an oil pump through the service valve so no air enters, in small amounts.
  ✓ Check: The level is back in the correct range after stable operation, and not above it.
6. Record. Record the oil type and quantity added and the measured oil pressure in the logbook.
  ✓ Check: The logbook shows what was done.
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.

Checks from the manufacturers’ manuals (19)

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

Copeland CoreSense Protection (Discus): one green flash warns of low oil pressure; after about 2 minutes of accumulated run time below roughly 7–9 psid it locks out and needs a manual or remote reset.^[2]
green ● · 0.5 s on, 0.5 s off · pause 2 s (timing assumed, not stated in the manual)
Dorin semi-hermetic compressors with oil pump: set the oil differential pressure switch to at least 0.8 bar with at least 90 s delay, so short dips at start-up do not stop the compressor.^[3]
Hussmann racks: oil pressure switches are preset to about 0.45 bar (6.5 psi) with 45 s delay for Carlyle and 0.6 bar (9 psi) with 120 s for Copeland; a current relay prevents false trips when the compressor is off.^[4]
Castel 5650E/5658E R744 oil level regulator: the red ALARM LED and the relay dropping out (contacts 1–3 closed) mean the level stayed low after several injection cycles; it resets itself once the level is back.^[5]
Carlyle 06D/06E/06CC: net oil pressure above suction should be about 1.2–1.8 bar (06D) or 1.2–2.3 bar (06E); the 120 s oil safety switch trips at about 8–10 psid net and needs a manual reset.^[6]
Carlyle 06D/06E/06CC: before adding oil after an oil switch trip, confirm oil is really lost and not trapped in the system or diluted by liquid; check levels after a defrost and keep 1/8–3/8 sight glass.^[6]
Copeland Stream 4M/6M: normal net oil pressure is 1–4.2 bar above crankcase pressure. Measure between the oil pump and a crankcase plug, not at the suction service valve. CoreSense warns below 0.95 bar for 4 s and locks out after 2 minutes below 0.95 bar; find the cause before the manual reset.^[7]
Bitzer CM-RC-01: after an oil warning the module locks out after a further 85 s (level switch OLC-D1) or 90 s (differential pressure switch DP-1); it also locks out after five identical stops within 24 hours or any five stops within one hour, and must then be reset after the cause is found.^[8]
Hussmann racks with Sporlan or AC&R oil level regulators: Mechanical oil level regulators work only if the unit and each compressor are level; do not pressure-test the floats above about 12 bar (175 psig). A compressor sight glass that is completely full of oil may mean a damaged regulator.^[9]
Danfoss condensing units: Suction gas velocity for oil return: at least 8–12 m/s in risers and 4 m/s in horizontal lines, horizontal lines falling about 0.5 % toward the compressor, and an oil trap (U-bend) at the foot of each riser with at most about 4 m between traps.^[10]
Copeland Scroll ZP*KW 25–40 ton: Read the oil level in a scroll's sight glass with the compressor stopped and oil equalised between manifolded compressors; more than about 40 mm below the glass centre is too low, while filling above 3/4 only pumps the surplus into the system. Above about 9 kg (20 lb) charge, extra oil is needed.^[11]
Hussmann transcritical CO2 racks: CO2 booster racks where the oil reservoir is vented to the flash tank: the flash tank pressure must stay at least about 5.5 bar (80 psi) above MT suction, otherwise oil does not flow back to the compressors. Also replace the oil separator when its pressure drop exceeds about 0.7 bar (10 psi).^[12]
Hussmann CO2 racks: Electronic oil level regulators only read correctly when the rack and each compressor stand level; a compressor sight glass completely full of oil can mean a damaged regulator that keeps injecting.^[12]
Heatcraft Beacon II systems with semi-hermetic compressors: After an oil pressure trip, check the suction superheat before adding oil: low superheat (liquid returning) dilutes the oil and makes the pressure drop. Also check the oil pickup screen in the sump and the auxiliary relay to the oil safety.^[13]
Copeland ZX/ZXL condensing units: After a motor burnout, the oil return orifice or screen in the suction accumulator is often clogged with debris and starves the new compressor of oil, giving a second failure. Replace the accumulator with the compressor; a suction filter drier of activated alumina may be fitted but must be removed after 72 hours.^[14]
Toshiba SMMS-i / SHRM-i VRF outdoor units: VRF with several outdoor units (Toshiba SMMS/SHRM-i): low oil error H07 is most often oil balance pipe valves left closed on one of the outdoor units in the line. The controller judges oil level by temperature difference between the oil-line sensors (TK1–TK3 minus TK4 at least 14 K means enough oil), so also check sensor positions and connectors; other causes are refrigerant in the compressor shell and a blocked oil return from the separator.^[15]
Toshiba SMMS-e MMY-MAP VRF: On VRF systems with several outdoor modules, a balance (oil-equalising) pipe valve left closed between the modules, or a leaking check valve or oil-equalising solenoid inside a module, starves one module's compressors of oil. Open all balance valves fully and test the oil circuit with nitrogen as the manufacturer describes.^[16]
Heatcraft MAC condensing units: Suction lines joined with 'bull-head' tees (two flows meeting head-on) or horizontal suction lines without fall towards the compressor (about 1/4 inch per 10 feet, roughly 2 mm/m) hold back oil.^[17]
Daikin VRV, VRV-III and VRV-WII systems (RXYQ, REYQ, RWEYQ) with FXxQ indoor units: VRV oil return runs automatically: the first time after 2 hours of cumulative compressor running from power-on, then at the latest after 8 hours, or earlier when the calculated oil loss (from Tc, Te and load) is high. In cooling the oil return stage lasts up to about 6 minutes on RXYQ/REYQ-M and ends early if the suction superheat (Ts − Te) falls below 5 K (9 °F); stopped and thermostat-off indoor units open their valves to about 200 pulses (192 on VRV-III), so refrigerant noise from a stopped unit during this time is normal.^[18]

📝 Fill in a service report

Calculate it yourself

HFC/HFO refrigerant pipe sizing →CO2 (R744) pipe sizing – copper →

Terms in this article

Liquid slugging

Low oil level or oil pressure alarm

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Check oil return and the oil separator

Leak test a refrigeration system

Check oil level and oil pressure

Test the crankcase heater

Check the pump-down function

Check oil return and the oil separator

Check oil return and the oil separator

Check oil level and oil pressure

Check oil level and oil pressure

Trace the control and safety chain

Checks from the manufacturers’ manuals (19)

Service report

Calculate it yourself

Terms in this article

Sources

Confirm the fault first

Troubleshoot step by step

Flowchart

Causes, most likely first

Check oil return and the oil separator

Leak test a refrigeration system

Check oil level and oil pressure

Test the crankcase heater

Check the pump-down function

Check oil return and the oil separator

Check oil return and the oil separator

Check oil level and oil pressure

Check oil level and oil pressure

Trace the control and safety chain

Checks from the manufacturers’ manuals (19)

Service report

Calculate it yourself

Terms in this article

Sources

Read also