F.25
Flue gas temperature too high
The boiler has detected an excessive flue-gas temperature condition.
What this code means
The boiler has detected an excessive flue-gas temperature condition.
Possible causes
- Circulation fault
- Pump, valve or blockage issue
- Wiring harness fault
- Flue gas temperature monitor triggered
Common symptoms
- F.25 displayed
- Boiler may lock out
Checks to make
- Do not attempt internal inspection
- Arrange diagnosis by a Vaillant installer or suitably qualified heating engineer
Safety
- Flue and combustion faults can create serious hazards. Do not operate or dismantle the boiler if a combustion or flue problem is suspected.
Model notes
Vaillant recommends professional investigation.