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Vaillant Boiler F.25 Error Code
Flue gas temperature too high on Vaillant boilers.
VaillantAll Vaillant boiler codes
The exhaust leaving the boiler is too hot, or the circuit to the flue gas safety cut-out has broken. Vaillant points to the cut-out itself (where one is fitted) and the wiring harness.
Compatible models
On Vaillant's UK fault code list (no models named) and in the ecoTEC plus VU ..6/6-5 OVZ manual, where it relates to an optional flue gas safety cut-out.
Also shown as
No other codes are listed for this fault.
Difficulty
Try the basic checks, but the usual fix is a technician's job.
Not sure you are reading the code right? See how to read Vaillant boiler codes.
How to fix Vaillant F.25
Switch the boiler off
Turn it off at the boiler and leave it. If you notice fumes or a flue smell, open the doors and windows wide and call an engineer.
Try one reset
After it has cooled, hold the reset button for more than 3 seconds. If the code returns, leave the boiler off.
When to call a technician
This one is about the exhaust side, so get a Gas Safe engineer out and don't use the boiler while the code keeps coming back. A carbon monoxide alarm sounding means everyone out and a call to the gas emergency line.
Parts linked to F.25
These are the parts an engineer usually checks for this code. They sit inside the sealed casing, so leave them to a Gas Safe registered engineer.
Frequently asked questions
Which Vaillant models show the F.25 code?
On Vaillant's UK fault code list (no models named) and in the ecoTEC plus VU ..6/6-5 OVZ manual, where it relates to an optional flue gas safety cut-out.
Can I fix Vaillant F.25 myself?
Only the outside checks. The steps above are the ones Vaillant's user instructions allow; if they don't clear F.25, the fix is inside the boiler and needs a Gas Safe registered engineer (RGII in Ireland).
Other Vaillant boiler codes
- F.22Water pressure in the heating system is too low
- F.28Boiler failed to light
- F.29Flame went out and the boiler couldn't relight
- F.20Overheat safety cut-out has tripped
- F.23Too big a gap between flow and return temperature
- F.24Temperature rising too fast
- F.27Flame detected with the burner off
- F.75No pressure rise when the pump starts
- F.00Flow or return temperature sensor not reading
- F.10Temperature sensor short circuit
- F.13Hot water cylinder sensor short circuit
- F.32Fan speed out of range
- F.61Gas valve control fault
- F.62Flame took too long to go out
- F.68Unstable flame
- F.70Invalid device code after a repair
- F.83Dry fire: no temperature change when the burner lit
Last reviewed 2026-10-21. Codes and steps can vary by model; check your owner's manual.