Vaillant Gas boiler fault F.77
Accessory fault (flue non-return flap, condensate pump, etc.)
Gas boilerWhat it means
Fault indicating a problem with an accessory such as the flue non-return flap or condensate pump, due to missing or incorrect feedback.
Likely causes
- No feedback from the flue non-return flap or feedback is incorrect
- Flue non-return flap defective
- No feedback from the condensate pump or feedback is incorrect
- Defective pump
Where to start
- Check that the flue non-return flap works without any problems.
- Check that the condensate pump is working correctly.
- 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 63), 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.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.42Flue non-return flap closed
- S.46Comfort protection: minimum load, loss of flame