A Daikin Altherma error code is a two-part code, such as 7H-01, that the unit shows when it stops or limits itself because of a fault. The first part is the main code and the second is a detail code that says which version of the fault happened. Always note both, because 7H-01 and 7H-04 point to different situations.
Codes show on the indoor unit's user interface and come from either the indoor or the outdoor unit. Some have simple causes such as a closed valve or a lost Wi-Fi link. Others involve the refrigerant circuit or the inverter, and Daikin says errors should be dealt with by a licensed installer before they are reset.
If a code returns, or you are not sure what it means, we can book a Daikin heat pump repair with an F-Gas certified engineer. For other makes, our heat pump error codes guide covers them by brand.
Safety first: isolate the power before going near the outdoor unit, and never remove the covers on either unit.
This guide is based on the Altherma 3 R W (ERGA outdoor unit with EHBH or EHBX indoor unit). Other Altherma models may differ, so check your own manual.
Please read first: this guide is here to help you understand what your Daikin Altherma heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official Daikin Altherma manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
If your Daikin Altherma heat pump is not showing a code, or you want to rule out the basics first, our guide to air source heat pump not working problems walks through the general checks and matches common faults to error codes across every major brand.
Is it safe to fix a Daikin Altherma fault yourself?
Only simple outside checks are safe for most homeowners. You can read the pressure gauge, check that valves you know about are open, clear snow or leaves from around the outdoor unit and check breakers.
Never open either unit to test sensors, circuit boards or wiring. The outdoor unit contains inverter electronics at mains voltage, and Daikin's own guide refers engineers to the service manual for these faults.
Refrigerant work, including any code linked to refrigerant pressure or charge, is legally restricted to F-Gas certified engineers. DIY repairs inside the unit can also affect your manufacturer warranty, so check your paperwork first.
Which Daikin error codes can you check yourself?
You can safely check the Daikin codes tied to water flow, pressure, airflow, hot water use and connections. Everything else needs an engineer.
| Code | What it means | Safe homeowner check | Stop and call an engineer if |
|---|---|---|---|
| 7H-01, 7H-04, 7H-05 | Water flow too low | Check any valves you closed are open and the pressure gauge reads above 1 bar. Clean the filter only if it is easy to reach and your manual says you can | It keeps coming back, or the filter is full of sludge |
| HJ-10 | Water pressure sensor reading abnormal | Read the gauge. If you have a filling loop and your manual allows it, top up to just above 1 bar | Pressure drops again or the gauge reads normal |
| E7-00, A5-00 | Outdoor fan fault, or high-pressure and freeze protection | Isolate the power, then clear leaves, snow or ice from the fan grille and coil | The unit is clear and the code returns |
| U8-01 | Lost connection with the LAN (Wi-Fi) adapter | Restart your router and check the adapter has power | Your internet works and the code stays |
| U8-02, U8-03 | Lost connection with the room thermostat | Replace the thermostat batteries if it has them | It still fails with fresh batteries |
| AJ-03 | Hot water took too long to heat | Wait for the next normal heat-up, as it resets itself | It keeps appearing on normal hot water use |
| U4-00, U5-00 | Units or user interface not communicating | Check breakers for both units, then restart once | It returns after the restart |
What are the most common Daikin Altherma error codes?
The Daikin codes we see most often are flow, freezing, pressure, communication and hot water faults. Daikin's guide describes each code but leaves fixes to its service manual, so causes labelled general engineering knowledge come from our own experience.
7H: water flow problem
7H is the most important indoor code family. Daikin says that if flow drops below 12 litres per minute, the unit stops for a while, shows 7H-01, then resets itself and carries on. The detail code tells you when it happened: 7H-04 during hot water, 7H-05 during heating, 7H-06 during cooling or defrost and 7H-07 while the pump is freeing itself.
As general engineering knowledge, closed valves, a blocked filter, air in the circuit or the wrong pump speed are typical, with sludge, scale or low pressure underneath. Check valves and pressure first. If the unit keeps stopping and starting, our guide to heat pump short cycling explains why that matters.
89-01, 89-02 and 89-03: heat exchanger frozen
All three mean the plate heat exchanger, where refrigerant passes heat to the water, has frozen. As general engineering knowledge, low flow in cold weather is the usual suspect.
Read how a normal heat pump defrost cycle works first. An engineer will check for air, sludge or a blocked filter and confirm the antifreeze protection settings.
HJ-10: water pressure sensor
HJ-10 means the water pressure sensor is reading something abnormal. It can be a wiring or sensor fault, but genuinely low pressure from a leak or a failed expansion vessel will also trigger it, as general engineering knowledge.
Check the gauge and top up only if you are allowed to. If it keeps falling, see our advice on a heat pump losing pressure.
E3-00, FA-00 and A5-00: high pressure
E3-00 means the outdoor high-pressure switch has tripped, and FA-00 is abnormal high pressure with the switch tripped. A5-00 covers high-pressure peak cut or freeze protection.
As general engineering knowledge, poor flow on the water side pushes refrigerant pressure up, so low flow is ruled out before the outdoor unit is blamed. These need an F-Gas engineer if the water side checks out.
U4-00, U5-00 and U8: communication faults
U4-00 means the indoor and outdoor units have lost contact, and U5-00 means the user interface has. U8-01 is a lost LAN adapter connection, while U8-02 and U8-03 are lost room thermostat connections.
For U4 and U5, an engineer checks the cabling and terminals. The U8 codes often have simple causes like a router restart or flat thermostat batteries.
E7-00: outdoor fan motor
E7-00 means the outdoor fan motor is not working properly. As general engineering knowledge, debris, ice or a wiring fault are the likely causes.
Isolate the power and clear anything around the fan. If it returns, the motor or its wiring needs an engineer.
Sensor faults
Several codes mean a temperature or pressure sensor has failed or is reading oddly. These include 80-00 and 81-00 (return and leaving water), H9-00 (outdoor air), C4-00 (heat exchanger), CJ-02 (room) and JA-00 (high pressure).
The usual fix is testing or replacing the sensor and its cable. There is no safe homeowner check.
AA, AC, AH and AJ: heater and hot water codes
AA-01 and AC-00 mean the backup or booster heater has overheated. AH-00 means the tank disinfection cycle did not finish, and Daikin advises scheduling it for a 4-hour window with no hot water use. AJ-03 means hot water took too long to heat and clears itself after a normal heat-up.
Daikin notes the booster heater can trip without showing a direct error. So if hot water takes ages, or AJ-03 or AH-00 keep appearing, ask an engineer to check it.
U0-00: refrigerant shortage
U0-00 means the unit has detected too little refrigerant. As general engineering knowledge, that usually means a leak or an undercharge at installation.
This is a job for an F-Gas engineer only. Do not keep resetting it, as running short of refrigerant can damage the compressor.
Which error code might you see in common situations?
What was happening when the code appeared narrows things down quickly. These are the Daikin Altherma codes most likely in everyday situations.
| Scenario | Codes you may see on a Daikin | First check |
|---|---|---|
| Cold snap, outdoor unit iced up | 89-01 to 89-03, A5-00, 7H-06, E7-00 | Allow time for a defrost, then clear snow. See our tips for a frozen outside unit |
| Losing pressure or low pressure | HJ-10, 7H-01 | Gauge above 1 bar, and drips from the relief valve pipe |
| Radiators lukewarm | 7H-05, E3-00, FA-00 | Valves open, then gurgling that suggests air |
| No hot water | 7H-04, AJ-03, AC-00 | Breakers for the backup and booster heaters |
| After a power cut | U2-00, E8-00, A1-00, U4-00, U5-00 | Breakers for both units, then one restart |
| Unit noisy or fan not spinning | E7-00, L4-00 | Debris or ice around the fan, power isolated |
| After a service or filter clean | 7H-01, 7H-05, HJ-10 | Valves reopened, air purged and pressure above 1 bar |
How do you reset a Daikin Altherma heat pump?
Daikin says errors should be dealt with by a licensed installer before they are reset, so a reset is a short-term step, not a fix. Some codes clear on their own: 7H-01 resets once flow recovers, and AJ-03 clears after a normal hot water heat-up.
For other codes, a homeowner can try one restart:
- Write down the full code, including the detail number after the dash.
- Switch the heat pump off at its breaker or isolator.
- Wait a few minutes, then switch it back on.
- Check the user interface. Your operation manual explains any menu option for acknowledging an error.
If the code comes back, leave it there. Resetting again and again without fixing the cause can put strain on the compressor and hides the pattern an engineer needs to see.
Daikin Altherma error code list
These are the codes listed in Daikin's Altherma 3 R W Installer's Reference Guide, grouped where they share a cause. Some descriptions were matched from an extracted copy of the guide, so confirm against your own manual.
Outdoor unit codes
| Code | Meaning | Who should fix |
|---|---|---|
| A1-00 | Zero cross detection problem (power supply) | Engineer |
| A5-00 | High-pressure peak cut or freeze protection | Homeowner check, then F-Gas engineer |
| E1-00 | Outdoor circuit board defect | Engineer |
| E3-00, FA-00 | High-pressure switch tripped, abnormal high pressure | F-Gas engineer |
| E5-00, E6-00 | Compressor motor overheated, compressor start-up defect | F-Gas engineer |
| E7-00 | Outdoor fan motor malfunction | Homeowner check, then engineer |
| E8-00, U2-00 | Power supply overvoltage or voltage defect | Engineer |
| EA-00 | Heating and cooling switchover problem | F-Gas engineer |
| F3-00, F6-00 | Discharge pipe temperature fault, high pressure in cooling | F-Gas engineer |
| H0-00, H8-00 | Voltage, current or compressor input sensor fault | Engineer |
| H3-00 | High-pressure switch malfunction | Engineer |
| H6-00 | Position detection sensor malfunction | Engineer |
| H9-00, J3-00, J6-00, J6-07, P4-00 | Outdoor air, discharge pipe, heat exchanger or fin temperature sensor fault | Engineer |
| JA-00 | High-pressure sensor malfunction | F-Gas engineer |
| L3-00 | Electrical box temperature rise | Engineer |
| L4-00 | Inverter fin temperature rise | Homeowner check, then engineer |
| L5-00 | Inverter overcurrent | Engineer |
| U0-00 | Refrigerant shortage | F-Gas engineer |
| U7-00 | Main and inverter processor communication fault | Engineer |
Indoor unit codes
| Code | Meaning | Who should fix |
|---|---|---|
| 7H-01, 7H-04 to 7H-07 | Water flow problem | Homeowner check, then engineer |
| 80-00, 81-00 | Return or leaving water temperature sensor fault | Engineer |
| 89-01 to 89-03 | Heat exchanger frozen | Engineer |
| 8F-00, 8H-00, 8H-03 | Outlet water temperature too high or water circuit overheating | Engineer |
| AA-01, AA-02, AC-00 | Backup, external backup or booster heater overheated | Engineer |
| AH-00 | Tank disinfection not completed | Homeowner check, then engineer |
| AJ-03 | Hot water heat-up took too long | Homeowner check, then engineer |
| C0-00, C0-01, C0-02 | Flow detected with pump off, or flow switch malfunction | Engineer |
| C4-00, CJ-02, H1-00, HC-00 | Heat exchanger, room, outdoor or tank temperature sensor fault | Engineer |
| EC-00, EC-04 | Abnormal tank temperature rise, tank preheating | Engineer if it persists |
| HJ-10 | Water pressure sensor abnormality | Homeowner check, then engineer |
| JA-17 | Refrigerant pressure sensor abnormality | F-Gas engineer |
| U3-00 | Underfloor screed dry-out not completed | Engineer |
| U4-00, U5-00 | Indoor to outdoor, or user interface, communication fault | Homeowner check, then engineer |
| U8-01 to U8-03 | Lost LAN adapter or room thermostat connection | Homeowner check, then engineer |
| UA-17, UA-21, UA-22 | Tank type, unit mismatch or option box communication fault | Engineer |
Where can you find the official Daikin Altherma manual?
Daikin publishes its installer reference guides as free PDFs, and the error code overview sits in the troubleshooting chapter. Check the model numbers on your units first, as newer E-series Altherma 3 R W units have their own guide.
- Daikin Altherma 3 R W installer reference guide, ERGA04 to 08DAV3 with EHBH/EHBX04 to 08DA (PDF)
- Daikin Altherma 3 R W installer reference guide, newer ERGA04 to 08EV with EHBH/EHBX E-series (PDF)
Need help with a Daikin Altherma fault?
We repair Daikin heat pumps across the UK, including the Altherma 3 range. Our engineers are F-Gas certified, so they can work on the outdoor unit and refrigerant circuit as well as the water side, and our repairs carry a 12-month guarantee.
We begin with a proper fault diagnosis and agree the repair with you before any work starts. If you have lost heating or hot water completely, our emergency heat pump repair team aims for same-day visits where possible. For anything less urgent, book a standard heat pump repair.
A large share of Daikin codes, 7H flow faults above all, start with a blocked filter, air or low pressure. Annual heat pump servicing deals with those before they stop the unit, and our cover plans spread the cost.
Call 0333 041 5130 or send us a message.
Frequently asked questions
Does a Daikin error code void my warranty?
No, a code on its own does not affect your warranty. What can cause problems is unqualified work inside the unit or a patchy service history, as manufacturer warranties usually depend on both. Keep your records and use a certified engineer for repairs.
Can I see Daikin error codes in an app?
If your system has a LAN or Wi-Fi adapter, faults may also show as alerts in the app. A U8-01 code means that adapter has lost its connection, so the app may go quiet even when the heat pump is fine. Our Daikin Altherma review looks at the range in more detail.
Does a compressor code mean the compressor has failed?
Not usually. Codes such as E5-00, E6-00 and L5-00 can trace back to supply voltage, poor flow or the inverter rather than the compressor itself. Our guide to heat pump compressor problems explains the signs of a real failure.
Why won't my Daikin compressor start when there is no error code?
Daikin lists several causes, including low water flow or volume, conditions outside the unit's operating range and settings linked to off-peak electricity tariffs. A signal from your energy supplier can also hold it off. If nothing is obvious, see our guide to a heat pump not working.
Why is water coming from the pressure relief valve?
Daikin's guide links a relief valve that opens to a broken expansion vessel, and one that drips to dirt blocking the valve outlet. Both need an engineer to test and, where needed, replace parts. Our advice on a heat pump leaking water covers other leak sources.
