F.25
FAULT CODE
Vaillant
Flue Gas Temperature Too High
High Severity
Engineer Required
The temperature of combustion gases in the flue has exceeded safe limits. This can indicate a blocked flue, incorrect flue installation, or a heat exchanger issue.
Cold Weather Note
In winter, birds or debris can partially block flue terminals. A Gas Safe engineer can inspect and clear these safely.
Step-by-Step Fix Guide
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Step 1
Do not attempt to inspect the flue yourself
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Step 2
Turn off the boiler
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Step 3 Engineer only
Call a Gas Safe engineer — the flue must be professionally inspected for blockages or incorrect installation
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.