F.731 on a Vaillant heat pump: High-pressure switch open
Short answer
On a Vaillant heat pump, F.731 means: High-pressure switch open. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: High-pressure switch open.
Part involved: High-pressure switch open.
When it shows: Integrated high-pressure switch in the outdoor unit trips (41.5 bar(g) per VWZ AI/split documents; 31.5 bar(g) per uniTOWER document).
Likely causes:
- Refrigerant pressure too high. The integrated high-pressure switch in the outdoor unit has tripped at 41.5 bar (g) or 42.5 bar (abs); Insufficient energy output via the condenser; Too low a volume flow as a result of closing single room controls in an underfloor heating system; Refrigerant flow rate too low (e.g. electronic expansion valve defective, 4-port diverter valve is mechanically blocked, filter is blocked). Inform customer service; Cooling mode: Check the fan unit for dirt.
On the flexoTHERM exclusive (VWF ../4)
The manual says: High-pressure switch open.
Part involved: High-pressure switch open.
Likely causes:
- Refrigerant pressure too high. The integrated high-pressure switch has tripped at 46 bar (g) or 47 bar (abs); Insufficient energy output via the relevant capacitors; Too low a volume flow as a result of closing single room controls in an underfloor heating system; Refrigerant flow rate too low (e.g. electronic expansion valve defective, 4-port valve mechanically blocked, filter blocked). Inform customer service.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: High pressure switch opened.
Part involved: High pressure switch opened.
Likely causes:
- Refrigerant pressure too high. The integrated high-pressure switch tripped at 41.5 bar (g); Insufficient energy output via the relevant capacitors; Insufficient volume flow as a result of closing individual room controls in an underfloor heating system; Refrigerant flow rate too low (e.g. electronic expansion valve defective, filter blocked).
Is it safe?
On the aroTHERM plus (VWL ../6 A)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - by the engineer only: reset the high-pressure switch and carry out a manual reset on the product.
On the flexoTHERM exclusive (VWF ../4)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Safety lockout |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
On the aroTHERM plus (VWL ../6 A)
- Note the full code and text shown on the display (and the date/time it appeared). Check radiator/underfloor valves are open and the system control is calling for heat. Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Contact your installer or Vaillant Service; do not attempt any repair.
On the flexoTHERM exclusive (VWF ../4)
- Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure). If you smell gas/solvent near the outdoor unit or hear hissing, switch the heat pump off at the isolator, keep ignition sources away and call an engineer.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Note the full code and text shown on the display (and the date/time it appeared). Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure). If you smell gas/solvent near the outdoor unit or hear hissing, switch the heat pump off at the isolator, keep ignition sources away and call an engineer.
What the engineer will check
On the aroTHERM plus (VWL ../6 A)
- Purging the building circuit; Check that the dirt filter that is in place is permeable; Check the high-pressure switch and high-pressure sensor; Reset the high-pressure switch and carry out a manual reset on the product.
Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).
Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.
On the flexoTHERM exclusive (VWF ../4)
- Purging the building circuit; Check that the dirt filter that is in place is permeable; VWL SA (cooling mode): Check the fan unit for dirt.
Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).
Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Purging the building circuit; Check the existing dirt filter for penetrability.
Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).
Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.
What you might notice
What owners and engineers report
19 owner and engineer reports mention this.
- No heating on the aroTHERM plus (VWL ../6 A): 8 reports
- Keeps stopping / cutting out on the aroTHERM plus (VWL ../6 A): 6 reports
- No hot water on the aroTHERM plus (VWL ../6 A): 5 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.35; uniTOWER VIH QW 190/6 E 0020291525_05 p.60; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.58 · Reviewed 23 September 2026
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.53; flexoTHERM 230 V 0020213395_04 p.48 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.38 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026