F.755 on a Vaillant heat pump: Fault: 4-port valve position not correct
Short answer
On a Vaillant heat pump, F.755 means: Fault: 4-port valve position not correct. 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 · 2 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Fault: 4-port valve position not correct.
Part involved: 4-port valve position not correct.
Likely causes:
- Incorrect position of the 4-port diverter valve. If, in heating mode, the flow temperature is lower than the return temperature in the building circuit; The temperature sensor in the electronic expansion valve environment circuit displays an incorrect temperature.
On the aroTHERM
The manual says: Fault: 4-port diverter valve position incorrect.
Part involved: 4-port diverter valve position incorrect.
Likely causes:
- Mechanical or electrical problem with the 4-port diverter valve.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| 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 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). The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant. 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 aroTHERM
- 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)
- Check the 4-port diverter valve (is an audible switching available? Use the sensor/actuator test); Check that the coil is positioned correctly on the 4-port valve; Check the wiring harness and plug connections; Check the temperature sensor in the electronic expansion valve environment circuit.
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 aroTHERM
- Move the 4-port valve from the control and check coil voltage; check harness/connections; check finned heat exchanger sensor plausibility (surface thermometer vs test menu T.0.28).
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
16 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 heating on the aroTHERM: 2 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.36; uniTOWER VIH QW 190/6 E 0020291525_05 p.62; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.59 · Reviewed 23 September 2026
- Installation instructions aroTHERM VWL 55/3 A 230 V, VWL 85/3 A 230 V (GB) ↗ · 0020217827_03 · page VWL ../3 A 0020217827_03 p.34; VWL ../2 A 0020147164_09 p.36 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026