Common Heat Pump Error Codes Explained

Heat pump error codes are designed to help you (and your technician) figure out what’s going wrong — but most of the time, they’re not easy to understand.

Each brand has its own codes, and the same code can mean different things depending on the system. Some are simple issues like a blocked filter. Others point to deeper faults like a compressor or sensor problem.

Below, we’ve listed some of the most common heat pump error codes by brand. If you see one of these codes on your system, the best thing to do is give us a call — we’ll check it properly and get it sorted fast.

Mitsubishi Ecodan Error Codes

Mitsubishi Ecodan systems are some of the most popular in Ireland, but like any heat pump, they can throw up error codes when something isn’t right. We often see U1 and P1 errors in older units — usually caused by blocked filters or low flow rates.

Below are some of the most common Mitsubishi Ecodan heat pump fault codes and what they mean:

Error Code	Description	Suggested Action
F3	Low pressure switch failure	Check the connection on the board and verify the continuity of the switch.
F5	High pressure switch failure	Inspect the connection on the board and ensure proper switch operation.
F9	Both pressure switch contacts are open	Examine both pressure switches for proper operation and wiring.
EA	Miswiring between FTC and Ecodan	Check S1, S2, S3 connections; verify voltages and communication lines.
EB	Miswiring between FTC and Ecodan	Similar to EA; inspect wiring and communication pathways.
EC	Unit cannot finish startup process	Generally caused by a communication fault; inspect system communications.
U1	High pressure fault	Often due to incorrect water flow rates; check with flow setter.
U2	High compressor discharge temperature	May be caused by refrigerant shortage or dirty condenser; inspect and rectify.
U3	Discharge thermistor problem	Typically involves TH4 discharge thermistor; check for open/close circuit.
U4	Thermistor circuit issue	Open or close circuit in thermistors (TH3, TH32, TH33, TH6, TH7, TH8); inspect connections.
U5	Inverter heat sink overheat protection	Reduced airflow through Ecodan; possibly faulty fan motor; inspect and address.
U6	Inverter/Compressor overcurrent	System indicates possible IPM error; perform diagnostic tests.
U8	Fan motor problem	Could be due to disconnected or obstructed DC fan motor; check connections and motor function.
U9	Overvoltage or undervoltage	Inspect power supply for voltage irregularities; stabilize as needed.
UD	Overheat protection	System has activated overheat protection; investigate cause and resolve.
UF	Compressor overcurrent or lock	Compressor may be locked; inspect and test compressor functionality.
P1	Flow temperature sensor fault	TH1 not connected, short or open circuit; verify sensor connections.
P6	Overheating at heat exchanger	Could be due to lack of water flow, air in the system, incorrect flow rate, blockage, or pump problem.
P8	No temperature change at plate heat exchanger	Indicates a potential issue with heat transfer; inspect exchanger and flow rates.
P9	TH5 sensor fault	Check the TH5 sensor for proper operation and connections.
PE	Inlet water temperature fault	Verify inlet water temperature readings and sensor functionality.

These codes usually show up on the main controller or display panel. Some can be cleared with a reset, but others need proper diagnosis to prevent long-term damage.

For a comprehensive list of Mitsubishi Ecodan error codes and detailed troubleshooting steps, you can refer to the Mitsubishi Electric Error Code Lookup Tool.

Daikin Heat Pump Error Codes

Daikin heat pumps are widely used across Limerick homes and businesses, known for their reliability and performance. Still, like any system, they can show error codes when something goes wrong. We often see U4 and A3 faults — usually caused by communication issues between units or a blocked drain line.

Below are some of the most common Daikin heat pump fault codes and what they mean:

Error Code	Description	Suggested Action
A1	Indoor unit PCB defect	Inspect and replace the indoor PCB if faulty.
A3	Drain level control system abnormality	Clear drain blockages and ensure proper drainage slope.
C4	Malfunction of liquid pipe thermistor	Check thermistor connections and replace if needed.
E3	Actuation of high-pressure switch	Clean outdoor heat exchanger and ensure proper water flow.
U0	Low refrigerant level	Check refrigerant levels and repair any detected leaks.
U2	Power supply insufficient or instantaneous failure	Verify power supply stability and inspect for voltage irregularities.
U4	Malfunction of transmission between indoor and outdoor unit	Inspect communication wiring and connections between units.
A5	Freeze-up protector activation	Ensure proper airflow and check for blocked filters or coils.
A6	Fan motor fault	Inspect fan motor operation and connections; replace if necessary.
C9	Faulty suction air temperature sensor	Verify sensor readings and replace the suction air temperature sensor if faulty.
E1	Outdoor unit PCB defect	Inspect and replace the outdoor PCB if defective.
E5	Compressor lock or service valves shut	Check compressor operation and ensure service valves are open.
E6	Compressor malfunction or overcurrent	Inspect compressor for faults and verify electrical connections.
E7	Fan motor malfunction or locked	Check fan motor for obstructions or defects; replace if necessary.
F3	Abnormal discharge pipe temperature	Inspect discharge pipe thermistor and ensure proper refrigerant flow.
H0	Current sensor faulty	Verify current sensor operation and replace if defective.
H3	High-pressure switch activation	Check for high-pressure conditions and inspect the high-pressure switch.
H6	Compressor startup failure	Inspect compressor startup sequence and electrical components.
J3	Malfunction of discharge pipe thermistor	Verify thermistor readings and replace if faulty.

For a comprehensive list of Daikin Altherma error codes and detailed troubleshooting steps, you can refer to the following resources:

Samsung Joule Heat Pump Error Codes

Samsung Joule air-to-water heat pumps are increasingly popular in Limerick homes for their efficiency and integration with Joule cylinders. However, like all systems, they can display error codes when issues arise. We often encounter E911 and E121 faults—typically related to water flow problems or communication errors between components.
Below are some of the most common Samsung Joule heat pump fault codes and their meanings:

Samsung Joule Error Codes

Error Code	Description	Suggested Action
E101	Communication error (Indoor/Outdoor)	Check communication cables and connections between indoor and outdoor units.
E121	Room temperature sensor error	Inspect the room temperature sensor for faults or disconnections.
E201	Flow switch error	Verify water flow; check for blockages or issues with the flow switch.
E202	Water pump malfunction	Ensure the water pump is operational; inspect for blockages or electrical issues.
E416	Water outlet temperature sensor fault	Check the outlet temperature sensor for proper operation and connectivity.
E911	Circulation problem detected	Inspect the system for airlocks, closed valves, or pump failures affecting circulation.
E177	Emergency stop activated	Determine the cause of the emergency stop; reset the system if necessary.
E403	Outdoor unit freezing detected	Check the outdoor unit for ice buildup; ensure defrost cycle is functioning correctly.

For detailed troubleshooting and additional error codes, please refer to the following resources:

Thermia Heat Pump Error Codes

Thermia heat pumps are a solid choice in Ireland, especially in rural homes with ground or air source systems. However, when something goes wrong, they can display error codes that may be challenging to interpret. We often see E101 and E201 errors—typically related to communication issues between indoor and outdoor units or sensor faults.
Below are some of the most common Thermia heat pump fault codes and their meanings:

Thermia Heat Pump Error Codes

Error Code	Description	Suggested Action
E101	Communication error between indoor and outdoor units	Check communication cables and connections between units.
E109	Incomplete indoor unit address	Verify and set the correct address for the indoor unit.
E111	Modbus communication error	Inspect Modbus connections and settings; ensure proper communication.
E162	EEPROM hardware error	Replace or repair the EEPROM hardware component.
E177	Emergency error	Determine the cause of the emergency stop; reset the system if necessary.
E201	Communication error between COMM KIT and outdoor unit	Check connections between COMM KIT and outdoor unit; ensure proper communication.
E202	Communication timeout between COMM KIT and outdoor unit	Inspect communication lines; verify that signals are being transmitted correctly.
E221	Outdoor unit temperature sensor error	Check the outdoor temperature sensor for faults or disconnections.
E231	Condenser temperature sensor error	Inspect the condenser temperature sensor; replace if faulty.
E251	Discharge temperature sensor error	Verify the discharge temperature sensor’s operation; replace if necessary.
E320	OLP (Overload Protector) sensor error	Check the OLP sensor for proper function; replace if defective.
E403	Outdoor unit compressor freezing detected	Inspect the compressor for freezing conditions; ensure defrost cycle is functioning correctly.
E404	Outdoor unit overload protection activated	Check for overload conditions in the outdoor unit; address any underlying issues.
E407	High-pressure condition detected	Inspect the system for high-pressure issues; ensure proper refrigerant levels and flow.
E416	Compressor discharge temperature too high	Verify refrigerant flow and compressor operation; address overheating causes.
E425	Power source line missing (3-phase models only)	Check the power supply lines for disconnections or faults; restore proper power.
E440	Heating operation blocked due to high outdoor temperature	Ensure outdoor temperature is within operational range; adjust settings if necessary.
E441	Cooling operation blocked due to low outdoor temperature	Verify outdoor temperature; adjust cooling settings to accommodate low temperatures.
E458	Outdoor unit fan 1 error	Inspect the outdoor unit’s fan for obstructions or faults; repair or replace as needed.
E461	Inverter compressor startup error	Check inverter and compressor startup sequence; address any issues preventing startup.
E462	Total current error or PFC overcurrent error	Inspect power factor correction components; ensure current levels are within acceptable ranges.
E463	Overload Protector (OLP) overheated	Allow the OLP to cool down; investigate causes of overheating and address them.
E464	Inverter IPM overcurrent error	Check the inverter’s Intelligent Power Module for overcurrent conditions; repair or replace if necessary.
E465	Compressor V limit error	Verify compressor voltage limits; ensure they are within specified parameters.
E466	DC link over/under voltage error	Inspect DC link voltage levels; stabilize voltage to acceptable range.
E467	Inverter compressor rotation error	Check compressor rotation; ensure it is operating correctly.
E468	Inverter current sensor error	Inspect the inverter’s current sensor; replace if faulty.
E469	Inverter DC link voltage sensor error	Verify the DC link voltage sensor’s operation; replace if necessary.
E470	Outdoor unit EEPROM read/write error	Check EEPROM for read/write functionality; repair or replace as needed.
E471	Outdoor unit EEPROM OTP error	Inspect the One-Time Programmable memory; address any errors detected.
E474	IPM or PFCM temperature sensor error	Check temperature sensors on IPM and PFCM; ensure they are functioning properly.

For detailed troubleshooting and additional error codes, please refer to the following resources:

Not Sure What the Error Code Means?
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If your heat pump is showing an error code, don’t guess — just give us a call. We’ll figure out what’s wrong and get it sorted, whether it’s a small sensor issue or something more serious.

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