Refrigeration & heat pump troubleshooting and error codes

Troubleshooting

Choose a machine type, click a component in the diagram or search for what you see, for example “trips on low pressure”. Each symptom has a flowchart with questions that lead to the likely cause and fix.

Field mode: large buttons, questions first and the screen kept on.

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

Symptoms

System trips on low pressureLow-pressure trips step by step: airflow and ice on the evaporator, low charge, blocked filter drier, expansion valve and low head pressure.System trips on high pressureHigh-pressure trips step by step: condenser fans and dirt, recirculating warm air, overcharge, air in the system and switch faults.System does not reach temperaturePoor cooling or heating step by step: controls and sensors, low suction pressure, a weak compressor, high load and high head pressure.Evaporator keeps icing upTroubleshoot an iced-up evaporator step by step: defrost, condensate drain, evaporator fan, moisture from doors and product, and low suction pressure.Compressor short cyclingShort cycling step by step: trips on low or high pressure or motor protection, small thermostat differential and control faults.Bubbles in the sight glassBubbles in the sight glass step by step: subcooling, blocked filter drier, unstable condensing pressure and low charge.High discharge temperature or hot compressorHigh discharge temperature step by step: high superheat, high compression ratio, motor faults and internal compressor leakage.Liquid floodback to the compressorLiquid floodback step by step: migration at standstill, badly mounted bulb, too little air and an oversized or badly set valve.Compressor will not startA compressor that will not start: alarms and safety devices, no demand, power supply faults and a seized or failed compressor.Heat pump outdoor unit ices upTroubleshoot ice on a heat pump outdoor unit step by step: defrost and four-way valve, meltwater drainage, airflow and low suction pressure.High standstill pressure in a CO2 systemHigh CO2 pressure at standstill or power failure: flash gas valve, high-pressure valve, gas cooler and safety valves.Compressor is noisy or vibratesA compressor that knocks, rattles or vibrates: liquid, lack of oil, loose mountings and pipes, overload and internal wear.Compressor motor protection tripsMotor protection trips: voltage and phase imbalance, overload, an overheated motor, too many starts and winding faults.Low oil level or oil pressure alarmLow oil level or oil pressure alarm: oil leaks, poor oil return, refrigerant in the oil, oil equalisation and the oil pump.Suction pressure too highSuction pressure too high: high load, an overfeeding expansion valve, leaking compressor valves, overcharge and a small compressor.Condensing pressure too lowCondensing pressure too low: cold air and fan control, low suction pressure, a cold receiver, leaking valves and settings.Superheat hunting (the valve hunts)A hunting expansion valve: bulb location, setting, flash gas before the valve, an oversized valve and the controller.Compressor runs but does not coolA compressor that runs without building pressure: wrong rotation, a stuck four-way valve, an empty system and internal leaks.Heat pump does not switch between heating and coolingA four-way valve that does not switch: control signal, coil, too little pressure difference, and a seized or leaking valve.Moisture, condensation or ice in the cold roomHumidity, condensation and ice in cold rooms: doors and seals, short run times, drainage, wet products and fans after defrost.Moisture indicator in the sight glass shows wetA moisture indicator showing wet: not yet settled, poor evacuation, water leaking in and a saturated filter drier.Safety valve discharges or leaksA safety valve that opens or leaks: CO2 standstill pressure, high operating pressure, a valve that does not reseat and set pressure.High receiver pressure (CO2)Rising receiver pressure in a CO2 booster: the flash gas valve, much flash gas in warm weather, MT compressors and sensors.Refrigerant detector alarmA gas detector alarm: a real leak, false alarms from other gases, a drifting sensor and a sensor in the wrong place.Filter drier or liquid line is cold or frostedFrost on the filter drier or liquid line: a clogged filter drier, a throttling valve and pressure drop in the line.Compressor runs constantlyA compressor that never stops: setpoint or sensor, low suction pressure, a compressor not pumping, high load and the evaporator.Flow switch or freeze protection tripsTroubleshoot a chiller flow switch or freeze protection stop: low flow, pump, strainer, a setpoint too low or weak glycol, low pressure and sensors.Refrigerant pump cavitates or loses pressureAn ammonia refrigerant pump that cavitates or loses pressure: too much flow, too little flow, vapour drawn in, low level and a blocked strainer.One evaporator does not cool, the others doTroubleshoot when one cold room or cabinet gets warm while the others on the same unit cool: controls, solenoid valve, ice, fan and expansion valve.The room gets too cold (solenoid valve does not close)A room that gets too cold: controls that do not cut out, a solenoid valve that does not close, a stuck relay and sensors.Ice cream machine does not freeze (product too soft)Soft ice cream that will not firm up: dispensing too fast, mix, condenser, feed pump, worn scrapers, belt and refrigerant.Ice machine makes little or no ice, or will not harvestToo little ice or ice that stays on the plate: water supply and pressure, condenser and room, harvest, ice pattern, leaking hot gas or water valves and scale.Ice cream machine freezes up (product too hard)Product too stiff or frozen in the cylinder: mix feed and priming, a frozen mix inlet, draw rate, consistency setting or brix, beater rotation and torque coupling.

Show the system diagrams – click a component to see its symptoms

Click a component in the diagram to see the symptoms related to it.

Direct expansion refrigeration systemCold rooms, display cases and condensing units with HFC, HFO or propane: compressor, condenser, receiver, filter drier, sight glass, expansion valve and evaporator.

Heat pump and air conditioningAir-to-air, air-to-water and split units with a four-way valve. The diagram shows heating mode: the indoor heat exchanger is the condenser and the outdoor unit is the evaporator.

CO2 booster (transcritical)Retail and industrial systems with R744: MT and LT compressors, gas cooler, high-pressure valve, receiver and flash gas valve.

Liquid chiller (water or brine)Chillers that cool water or brine in a plate heat exchanger: compressor, condenser, filter drier, expansion valve, evaporator, brine pump and flow switch.

Ammonia with pump circulation (R717)Industrial refrigeration with ammonia: compressor, oil separator, condenser, receiver, level control, low pressure receiver, refrigerant pump and evaporator.

Cold rooms with several evaporatorsOne condensing unit serving several cold rooms or cabinets: each evaporator has a solenoid valve and an expansion valve.

Soft ice cream machineSoft-serve and ice cream machines with a freezing cylinder: compressor, condenser, filter drier, expansion device, freezing cylinder with beater and belt, and mix hopper with pump.

Ice machineCube, flake and nugget ice machines: compressor, condenser, filter drier, expansion valve, evaporator (freeze plate or freezing cylinder), hot gas valve for the harvest, water pump, water inlet valve and water sump.

Discharge gasLiquidLiquid and vapour after the expansion valveSuction gasWater or brine

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.

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