F.25
Vaillant
Flue Gas Temperature Too High / Flue Gas Sensor Trip
Boiler shows F.25, burner shuts down, possible flue or fan noise
Advertisements
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Advertisements
Possible Causes
Blocked or restricted flue, Faulty flue gas temperature sensor, Fan not running at correct speed, Incorrect flue installation
How to Fix / Troubleshooting
Safety: This fault can indicate unsafe combustion. Do not operate the boiler if flue blockage is suspected. Contact a Gas Safe engineer immediately.
- Visual flue check: From outside, ensure the flue terminal is not blocked by debris, snow, or vegetation. Do not modify the flue yourself.
- Reset once: If no obvious blockage, you may attempt a single reset. If F.25 returns, switch off the boiler and call an engineer.
- Engineer tasks (for reference only): A technician will check the flue gas sensor, verify fan speed via diagnostics, inspect the flue for internal collapse or separation, and confirm combustion with a flue gas analyser.
Advertisements
Repair Difficulty
Professional Required
5/5
Required Part
Flue Gas Temperature Sensor
Check Price on Amazon
We may earn a commission from links on this page.
Advertisements