Vaillant Gas boiler fault F.84
Temperature difference fault at the flow and return temperature sensors
Gas boilerWhat it means
Fault indicating an abnormal temperature difference reading between the flow and return temperature sensors, typically due to incorrect sensor installation.
Likely causes
- Flow NTC installed incorrectly
- Return NTC installed incorrectly
- Defective heating flow or return temperature sensor
Where to start
- Check that the flow NTC has been installed correctly.
- Check that the return NTC has been installed correctly.
- Pull out the heating flow (4) and return temperature sensor (3)
The full answer, free
- 1 more check 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.77Accessory fault (flue non-return flap, condensate pump, etc.)
- F.83Flow and/or return temperature sensor temperature change fault
- 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