Vaillant Gas boiler fault S.42
Flue non-return flap closed
Gas boilerWhat it means
Flue non-return flap return signal blocks burner operation (only in conjunction with the multi-functional module) or condensate pump defective, heat demand is blocked.
Likely causes
- Flue non-return flap signal blocking operation
- Condensate pump defective
- Defective pump
Where to start
- Check flue non-return flap signal (with multi-functional module)
- Check condensate pump operation
- Pull out the plug (5) for the pump
The full answer, free
- 4 more checks in order
- How to put it right, step by step
- The manual page for this fault (page 59), right under the answer
Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.
From the Vaillant manual for the Gas boiler. Check the model on the data plate before you order parts.
Other Vaillant Gas boiler fault codes
- F.00Flow temperature sensor interruption
- F.01Return temperature sensor interruption
- F.03Cylinder temperature sensor interruption
- F.10Flow temperature sensor short circuit
- F.11Return temperature sensor short circuit
- F.13Cylinder temperature sensor short circuit
- F.22Safety shutdown: water deficiency
- F.23Safety shutdown: temperature spread too great
- F.24Safety shutdown: temperature rise too fast
- F.25Safety shutdown: flue gas temperature too high
- F.27Safety shutdown: flame simulation
- F.32Fan fault
- F.33Air pressure switch fault
- F.64Fault: electronics/NTC
- F.65Electronics temperature fault
- F.72Flow and/or return temperature sensor fault
- F.74Water pressure sensor signal outside correct range (too high)
- F.75Pump fault/water deficiency
- F.77Accessory fault (flue non-return flap, condensate pump, etc.)
- F.83Flow and/or return temperature sensor temperature change fault
- F.84Temperature difference fault at the flow and return temperature sensors
- F.85Flow and return temperature sensors installed incorrectly (inverted)
- P.02Minimum load check programme
- S.46Comfort protection: minimum load, loss of flame