It is a flue fault, so treat it seriously. Leave the boiler off, make sure your carbon monoxide alarm works, and book an engineer.

F.25

Vaillant fault code

Vaillant F.25: the flue gas temperature is too high

Engineer only

The gases leaving through the flue are hotter than they should be. Because this involves the flue, it is a job for an engineer only.

What you can safely do

There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.

Book a Gas Safe registered engineer.
If anyone feels unwell (headache, dizziness, nausea), get fresh air and call 0800 111 999.

Find a Gas Safe engineer near you Opens the official Gas Safe Register search.

Applies to: ecoTEC plus. Codes can differ between models, so check your manual.

What F.25 means on each Vaillant model

From Vaillant's own fault tables. Your model name is usually on the front of the boiler or in the user manual.

Model	Meaning	Causes listed for engineers
ecoTEC plus	Safety switch-off: flue gas temperature too high	Break in the plug connection for the optional flue gas safety temperature limiter, break in cable harness