A Vaillant heat pump error code is the unit telling you it has stopped, or held itself back, to protect a part. On aroTHERM plus and aroTHERM split models the code shows on the controller screen as an F and a number, such as F.022, with a short line of text. You may also see plain messages like "Heating compressor blocked" with no number at all.
Most codes come down to a few causes: low water pressure, poor flow, a blocked fan, a defrost problem or a lost link between the indoor and outdoor units. Some you can check safely in a few minutes. Others involve mains electrics or the sealed refrigerant circuit, and those are for a qualified engineer only.
If a code keeps coming back, or you are not sure what it means, stop there. We can book a Vaillant heat pump repair with an F-Gas certified engineer, often on the same day. For other makes, our heat pump error codes guide covers them by brand.
Safety first: switch off at the isolator before going near the outdoor unit, and never remove its covers.
The Glow-worm GeniaAir, built by the same group, uses the same F codes, so this guide applies to it too.
Please read first: this guide is here to help you understand what your Vaillant 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 Vaillant 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 Vaillant aroTHERM 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 walks through the general checks and matches common faults to error codes across every major brand.
Is it safe to fix a Vaillant heat pump fault yourself?
Only the basic checks are safe for most homeowners. That means reading the pressure gauge, clearing leaves or snow from around the outdoor unit, checking breakers and the isolator, and resetting the system once.
Never open the outdoor unit or the indoor hydraulic station to look at wiring. The inverter electronics run at mains voltage and can stay live for a while after you switch off. Some aroTHERM models also use R290 refrigerant, which is flammable.
Any work on the refrigerant circuit is legally restricted to F-Gas certified engineers. Manufacturer warranties also tend to rely on proper installation and servicing, so a DIY repair inside the unit can put your cover at risk. Check your paperwork before going further than the checks below.
Which Vaillant error codes can you check yourself?
You can safely check a handful of Vaillant codes linked to pressure, airflow, defrost, flow and power. Everything else in this guide needs an engineer.
| Code | What it means | Safe homeowner check | Stop and call an engineer if |
|---|---|---|---|
| F.022 | Heating water pressure too low | Read the gauge. If your system has a filling loop and your manual allows it, top up to the pressure your installer set | It drops again within days, or you can see a leak |
| F.718, F.785 | Fan not confirming it is turning | Switch off at the isolator, then clear leaves, snow or debris from the grille and around the unit | The fan is clear and the code returns |
| F.733 | Evaporation temperature too low, often weak airflow | Make sure the outdoor coil is not clogged with leaves or covered by snow | Airflow is clear and it trips again |
| F.741, F.283 | Defrost problem | Check it is not a normal defrost (these last roughly 2 to 10 minutes) and that the condensate drain is not blocked or frozen | Ice keeps building on the unit |
| F.819 | Inverter overheating | Let the unit cool and check nothing is blocking air around it | It happens more than once |
| "Compressor blocked" message | Compressor held off, often a flow problem | Check any heating valves you closed are open again. Clean the system strainer only if it is easy to reach and your manual says you can | Flow does not recover, or the job means draining down |
| F.9998 | Indoor and outdoor units have lost contact | Check the isolator and breakers are on, then power cycle once | It comes back after a restart |
If a breaker has tripped, reset it once. If it trips again, leave it off and call us, as that points to an electrical fault.
What are the most common Vaillant heat pump error codes?
The Vaillant codes we see most often cover low pressure, flow, fans, defrost and communication. Published code tables give the meanings below; causes marked as general engineering knowledge come from our own experience, not a Vaillant manual.
F.022: low water pressure
F.022 means the pressure in the heating circuit has dropped too low. Published causes are leaks, trapped air or a faulty pressure sensor. From general engineering knowledge, a tired expansion vessel or a weeping relief valve can also let pressure fall.
Top up only if you have a filling loop and the manual allows it. If it falls again, read our guide to a heat pump losing pressure, then book an engineer to find the leak and test the vessel and sensor.
F.788, F.820 and F.094: pump and flow faults
F.788 means the heating circuit pump has reported a fault, and F.820 means the pump has stopped signalling back. F.094 is a fault with the flow sensor. As general engineering knowledge, sludge, a blocked strainer, an air lock or a closed valve can all starve the system of flow.
Check valves you know about are open and the pressure is normal. After that, an engineer needs to test the pump and wiring. Older systems full of sludge may need a heat pump powerflush.
F.718 and F.785: fan blocked
These mean the outdoor unit cannot confirm that fan 1 (F.718) or fan 2 (F.785) is turning. The published fix is to clear the airflow path. In general, leaves, snow or ice on the coil are the usual culprits.
Switch off at the isolator before clearing anything by hand. If the fan is clear and the code returns, the motor or its wiring needs an engineer.
F.741 and F.283: defrost faults
F.741 means the return water fell below 13°C while the unit was de-icing, which published tables link to too little water in the system. F.283 means de-icing failed and comes from the Glow-worm GeniaAir list. From general engineering knowledge, a frozen condensate drain can also stop a defrost finishing.
Read how a normal heat pump defrost cycle looks before you worry. If ice keeps building, an engineer will check the system volume and backup heater.
F.731, F.733 and F.823: refrigerant protection
F.731 means the high pressure switch has opened. F.733 means evaporation temperature is too low, usually from weak airflow, and F.823 means the hot gas thermostat has tripped. A forum thread links F.733 to a dirty fan unit and a sticking expansion valve.
As general engineering knowledge, an air lock or blocked filter on the water side can trip F.731 too. Clear any debris from the coil, then call an F-Gas engineer, because the next checks involve the refrigerant circuit.
F.752, F.753 and F.817 to F.819: inverter faults
These point to the inverter board that drives the compressor. F.753 means the main board cannot see the inverter, F.818 means the mains voltage is wrong and F.819 means the inverter is overheating.
One owner reported F.753 on a unit only three weeks old, so it is not just an old-age fault. Leave the electrics alone and book an engineer.
Temperature and pressure sensor faults
Codes such as F.514, F.519, F.520, F.585, F.703 and F.825 mean a sensor is unplugged or shorted. One owner who had F.519, F.520, F.585 and F.703 together found the fault was the wiring harness or main board, cured by a board swap.
There is no homeowner fix here. An engineer will test the sensor, plug and harness.
F.9998 and F.9997: communication faults
F.9998 means the indoor and outdoor units have lost contact over the eBUS cable. Published causes are a missing, loose, reversed or undersized cable, or interference from nearby mains wiring. F.9997, from the Glow-worm list, is a protocol mismatch between the two units.
Power cycle once. If it returns, an engineer will check the cable, which Glow-worm says should run at least 120 mm from mains cables.
"Compressor blocked" messages
This text often appears with no F code. One installer on a heat pump forum puts most hot water "compressor blocked" call-outs down to blocked filters, though that is opinion rather than a Vaillant figure.
An engineer will clean the strainers, check flow in the test menu and clear the fault list.
Which error code might you see in common situations?
The situation often tells you more than the code alone. Here is what a Vaillant unit tends to show in everyday problems, and what to look at first.
| Scenario | Codes you may see on a Vaillant | First check |
|---|---|---|
| Cold snap, outdoor unit iced up | F.741, F.283, F.733, F.718, F.785 | Wait 10 minutes in case it is defrosting, then check the drain and clear snow. See our tips for a frozen outside unit |
| Losing pressure or low pressure | F.022, F.073 | Read the gauge and look for drips at valves and radiators |
| Radiators lukewarm | F.022, F.788, F.820, F.094, "Heating compressor blocked" | Pressure, then radiator and isolation valves |
| No hot water | "DHW compressor blocked" | Strainer, if your manual allows you to clean it |
| After a power cut | F.9998, F.818, F.764, F.1117 | Breakers and isolator on, then one restart |
| Noisy, or fan not spinning | F.718, F.785, F.819 | Debris or ice around the fan grille |
| After a service or filter clean | F.022, F.094, F.788, F.9998 | Pressure topped back up and all valves reopened |
How do you reset a Vaillant heat pump?
You reset a Vaillant heat pump by switching it off at the isolator, waiting about five minutes and switching it back on. Before you do, write down the code or take a photo of the screen.
- Note the F code and any message on the controller.
- Turn the heat pump off at its isolator switch, usually on the wall near the outdoor unit.
- Wait around five minutes.
- Switch it back on and give it time to restart before judging whether it worked.
Some faults are stored and stay in the fault list until an engineer clears them from the installer menu. So a code that seems to linger after a reset is not always a fresh fault.
One reset is fine. Resetting again and again without fixing the cause can stress the compressor, and it hides the pattern an engineer needs to see. If the code returns, book a visit.
Vaillant error code list
This list covers 42 aroTHERM F codes, taken from published third-party tables rather than a Vaillant manual. Codes marked GW come from the Glow-worm GeniaAir list, so check your own manual for those.
| Code | Meaning | Who should fix |
|---|---|---|
| F.022 | Heating circuit pressure low | Homeowner check, then engineer |
| F.042 | Coding resistor fault | Engineer |
| F.073 | Heating circuit pressure sensor fault | Engineer |
| F.094 | Flow sensor fault | Engineer |
| F.103 | Wrong control board fitted in outdoor unit | Engineer |
| F.283 (GW) | De-icing unsuccessful | Homeowner check, then engineer |
| F.514, F.517 | Compressor inlet or outlet temperature sensor fault | Engineer |
| F.519, F.520 | Heating return or flow temperature sensor fault | Engineer |
| F.526, F.585, F.774, F.825 | Expansion valve outlet, condenser outlet, air inlet or refrigerant inlet sensor fault | Engineer |
| F.546, F.703 | High or low pressure sensor fault | Engineer |
| F.572 (GW) | Inverter board fault | Engineer |
| F.582 | Electronic expansion valve fault | F-Gas engineer |
| F.718, F.785 | Fan 1 or fan 2 blocked | Homeowner check, then engineer |
| F.729, F.732 | Compressor outlet temperature too low or too high | F-Gas engineer |
| F.731 | High pressure switch open | F-Gas engineer |
| F.733 | Evaporation temperature too low | Homeowner check, then F-Gas engineer |
| F.734, F.735, F.737 | Condensation or evaporation temperature out of range | F-Gas engineer |
| F.739 (GW) | Low refrigerant volume | F-Gas engineer |
| F.741 | Return below 13°C during de-icing | Homeowner check, then engineer |
| F.752, F.753, F.817 | Inverter (frequency converter) fault or not recognised | Engineer |
| F.755 | Four-port valve in wrong position | F-Gas engineer |
| F.764 (GW), F.1117 | Compressor phase error or failure | Engineer |
| F.788, F.820 | Heating pump fault or not signalling back | Engineer |
| F.818 | Wrong mains voltage | Engineer |
| F.819 | Inverter overheating | Homeowner check, then engineer |
| F.823 | Hot gas temperature switch open | F-Gas engineer |
| F.9997 (GW) | Indoor and outdoor protocol mismatch | Engineer |
| F.9998 | Indoor to outdoor connection fault | Homeowner check, then engineer |
Where can you find the official Vaillant manual?
Vaillant publishes its installation and operating instructions as free PDFs on its UK professional site. On aroTHERM plus systems the fault code table sits in the indoor appliance interface instructions, while aroTHERM split codes are in the hydraulic station manual.
- VWZ AI heat pump appliance interface instructions for aroTHERM plus (PDF)
- aroTHERM plus VWL 35 to 75/6 A S2 installation and operating manual (PDF)
- aroTHERM split hydraulic station VWL 57 to 127/5 IS instructions (PDF)
- Vaillant literature and manuals finder
Need help with a Vaillant heat pump fault?
We fix Vaillant heat pumps across the UK, from aroTHERM plus to older split systems. Our engineers are F-Gas certified, so they can work on the refrigerant side as well as the water and electrics, and every repair carries our 12-month guarantee.
We start with proper fault diagnosis, then quote for the repair before any work goes ahead. If your heating or hot water has stopped altogether, our emergency heat pump repair team aims for same-day visits where we can. For anything less urgent, book a standard heat pump repair.
Many of the codes above, especially flow, pressure and fan faults, are avoidable. Regular heat pump servicing catches clogged strainers, low pressure and dirty coils before they trip the unit, and our cover plans spread the cost.
Call us on 0333 041 5130 or get in touch online.
Frequently asked questions
Does a Vaillant error code void my warranty?
No, a code on its own does not affect your warranty. Trouble tends to come from unqualified work inside the unit or missed servicing, as most manufacturer warranties depend on both. Keep your service records and use a certified engineer for repairs.
Can I see Vaillant error codes in an app?
Depending on your controller and whether an internet gateway is fitted, faults may also show as alerts on your phone. The controller screen is still the most reliable place to read the full F code. Our Vaillant aroTHERM review looks at the range in more detail.
Does a compressor code mean the compressor has failed?
Usually not. Codes such as F.817 and F.1117 often trace back to the power supply, a blown fuse, wiring or the inverter board. Our guide to heat pump compressor problems explains the signs of a genuine failure.
Are old error codes stored on the controller?
Yes, Vaillant heat pumps keep a fault list, and some entries stay there until an engineer clears them. That history helps us spot a repeat problem quickly, so a photo of the screen each time a code appears is useful too.
How long does a Vaillant heat pump repair take?
Many faults, such as a blocked strainer or a loose eBUS connection, are fixed on the first visit. Parts like a pump, sensor harness or inverter board may need ordering, which adds time. We will tell you what to expect once the fault is diagnosed.
