Vaillant heat pump error codes

17 codes that Vaillant publishes for its heat pumps, each with what it means, the likely cause, what you can safely check and when to call an installer. Tap your code:

Heat pump: do only the checks the maker gives the owner for that code — read the water pressure on the controller, keep the outdoor unit clear of leaves and snow, switch the power off and on where the maker says to. Keep the casing on: refrigerant, wiring and the water circuit’s parts are not owner work. Anything else is for the heat-pump installer or the maker’s service engineer: in the UK and EU only an F-gas certified technician may open the refrigerant circuit. Codes that need one are marked Installer.

Read this first

Vaillant aroTHERM plus heat pumps show a fault as F. and three digits — F.022, F.718, F.788 — on the display of the heat pump’s control module (the VWZ AI appliance interface). The codes here are from the VWZ AI operating and installation instructions: the operating part tells the owner what to check and when to call a competent person, and the installation part’s fault table names what the installer checks. The aroTHERM plus’s own operating instructions add how to keep the outdoor unit clear. Vaillant’s older aroTHERM (VWL ../3) uses many of the same numbers. Models differ: your model’s manual is the authority. An S. code (S.516 de-icing, S.312) is a status message, and an M. code a maintenance message, not a fault.

`F.022` Heating water pressure too low

What it means: The water pressure in the building (heating) circuit is too low. The display alternates F.22 and the current pressure once it has been below 0.5 bar for more than a minute; below 0.7 bar the maintenance message M32 shows first.
Likely cause: Water lost through a leak or air pockets in the heating circuit, or a faulty pressure sensor.
What you can safely check: Heat pump: do only the checks the maker gives the owner for that code — read the water pressure on the controller, keep the outdoor unit clear of leaves and snow, switch the power off and on where the maker says to. Keep the casing on: refrigerant, wiring and the water circuit’s parts are not owner work.
Read the pressure: Menu → Live Monitor → Water pressure. Vaillant asks for 1.0 to 1.5 bar with the system cold (more in a house of several storeys).
If the pressure drops often, the reason needs finding: tell your competent person (installer).
When to call an installer: Call your installer to top up the heating water and find the leak; the VWZ AI instructions leave filling to the competent person.

Source: Vaillant, Heat pump appliance interface VWZ AI — Operating instructions and Installation instructions (0020291573_01, 14.12.2022): 6 Troubleshooting, appendix F Fault codes Checked 2026-09-29.

`F.723` Building circuit pressure too low

What it means: The filling pressure has fallen below the minimum and the heat pump has switched itself off.
Likely cause: Water lost from the heating circuit.
What you can safely check: Heat pump: do only the checks the maker gives the owner for that code — read the water pressure on the controller, keep the outdoor unit clear of leaves and snow, switch the power off and on where the maker says to. Keep the casing on: refrigerant, wiring and the water circuit’s parts are not owner work.
Read the pressure: Menu → Live Monitor → Water pressure.
When to call an installer: The operating instructions say to inform your competent person — for a heat pump, your installer — so they can top up the heating water.

`F.718` `F.785` Outdoor fan blocked

What it means: The heat pump has no signal that the outdoor unit’s fan is turning: F.718 fan 1, F.785 fan 2.
Likely cause: Something blocking the fan or its air route; the installer’s remedy is to check the air route and remove blockages.
What you can safely check: Heat pump: do only the checks the maker gives the owner for that code — read the water pressure on the controller, keep the outdoor unit clear of leaves and snow, switch the power off and on where the maker says to. Keep the casing on: refrigerant, wiring and the water circuit’s parts are not owner work.
Keep the outdoor unit clear, as the aroTHERM plus operating instructions ask: remove branches, leaves and snow gathered around it, and snow on its air inlet and outlet grilles. Keep the casing on.
When to call an installer: If the code returns with the unit clear, call your installer.

Source: Vaillant, aroTHERM plus VWL 35/6 A 230V S2 … VWL 75/6 A S2 — Operating instructions and Installation and maintenance instructions (0020330791_03, 06.11.2023): 6 Care and maintenance Checked 2026-09-29.

`F.733` Evaporation temperature too low

What it means: The refrigerant is evaporating at too low a temperature.
Likely cause: In heating mode, too little air moving through the outdoor unit’s heat exchanger, or too little refrigerant.
What you can safely check: Heat pump: do only the checks the maker gives the owner for that code — read the water pressure on the controller, keep the outdoor unit clear of leaves and snow, switch the power off and on where the maker says to. Keep the casing on: refrigerant, wiring and the water circuit’s parts are not owner work.
Keep the outdoor unit’s grilles free of leaves, dirt and snow, as the aroTHERM plus operating instructions ask.
When to call an installer: If the code returns, call your installer: the refrigerant volume is F-gas certified work.

`F.788` Heating circuit pump fault

What it means: The high-efficiency pump has detected a fault (dry running, blockage, over- or undervoltage), switched off and locked.
Likely cause: The pump or its supply; the installer’s steps include switching the heat pump off for at least 30 seconds, purging the circuit and checking the dirt filter.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.741` Return water too cold during defrost

What it means: During de-icing the return temperature of the heating circuit fell below 13 °C; the compressor is blocked until it rises above 20 °C.
Likely cause: Too little water volume in the system to supply the de-icing heat.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer: Vaillant’s remedies are making sure the system holds its minimum water volume — if needed with a series return cylinder — and switching on the electric back-up heater.

`F.731` High-pressure switch open

What it means: The refrigerant pressure went too high and the high-pressure switch tripped (at 41.5 bar gauge).
Likely cause: Too little heat taken away by the heating water: air in the circuit, closed room controls on underfloor heating, a blocked dirt filter, or a refrigerant-flow fault.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.732` Compressor outlet temperature too high

What it means: The compressor outlet temperature rose above 110 °C.
Likely cause: Operating limits exceeded, an expansion valve not opening correctly, or too little refrigerant.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer: the refrigerant checks are F-gas certified work.

`F.737` Condensation temperature too high

What it means: The temperature on the heating side is too high for the compressor to run.
Likely cause: Heat fed into the heating circuit from elsewhere, too little flow in the heating circuit, or too much refrigerant.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.752` Frequency converter fault

What it means: An internal electronics fault on the outdoor unit’s inverter board, or mains voltage outside 70–282 V.
Likely cause: The inverter board, its cables, or the mains voltage (which should be 195–253 V).
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.818` Mains voltage wrong for the inverter

What it means: The mains voltage is wrong for running the frequency converter.
Likely cause: A supply problem — Vaillant also lists a shutdown by the electricity supplier.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer or an electrician to measure the supply.

`F.819` Inverter overheating

What it means: The frequency converter has overheated inside.
Likely cause: Its air route or fan, or an outdoor temperature above the unit’s 46 °C limit.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.755` Four-port valve in the wrong position

What it means: The 4-port diverter valve that switches between heating and cooling is in the wrong position (in heating, flow colder than return).
Likely cause: The valve, its coil, its wiring, or a temperature sensor giving a wrong reading.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.519` `F.520` Heating circuit temperature sensor fault

What it means: The return (F.519) or flow (F.520) temperature sensor of the heating circuit is not connected or short-circuited.
Likely cause: The sensor or its wiring harness.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

`F.073` Water pressure sensor fault

What it means: The heating-circuit water pressure sensor is not connected or short-circuited.
Likely cause: The sensor or its wiring harness.
What you can safely check: Nothing for the owner to check: this one is for an installer.
When to call an installer: Installer Call your installer.

A code that is not listed here

The VWZ AI instructions list more than 30 F. codes. For any code not listed here, the operating instructions say to contact a competent person: for a heat pump that is your installer. If “Lhm. 37” shows, the heat pump has found a permanent fault and is running in comfort-protection mode with restricted comfort — Vaillant says to consult a competent person.

We list only codes the source below states. If your code is not here, it may belong to a model the source does not cover: your model’s manual is the authority.

Source

Every code on this page comes from these pages, published by Vaillant and read on the date shown. Meanings are written in our own words; nothing is taken from other makes, other models or third-party code sites.

Vaillant: Heat pump appliance interface VWZ AI — Operating instructions and Installation instructions (0020291573_01, 14.12.2022): 6 Troubleshooting, appendix F Fault codes Checked 2026-09-29.
Vaillant: aroTHERM plus VWL 35/6 A 230V S2 … VWL 75/6 A S2 — Operating instructions and Installation and maintenance instructions (0020330791_03, 06.11.2023): 6 Care and maintenance Checked 2026-09-29.

Vaillant heat pump error codes

Read this first

F.022 Heating water pressure too low

F.723 Building circuit pressure too low

F.718 F.785 Outdoor fan blocked

F.733 Evaporation temperature too low

F.788 Heating circuit pump fault

F.741 Return water too cold during defrost

F.731 High-pressure switch open

F.732 Compressor outlet temperature too high

F.737 Condensation temperature too high

F.752 Frequency converter fault

F.818 Mains voltage wrong for the inverter

F.819 Inverter overheating

F.755 Four-port valve in the wrong position

F.519 F.520 Heating circuit temperature sensor fault

F.073 Water pressure sensor fault