Boiler shows F25 and shuts down, sometimes with signs of poor combustion

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.

Possible Causes

Blocked flue or terminal, Faulty flue gas temperature sensor, Incorrect flue installation, Recirculation of combustion products

Safety first: Turn off power and gas. Risk of carbon monoxide if flue is compromised.

Steps to check:

Inspect the external flue terminal for obstructions or damage.
Ensure that any flue joints visible near the boiler are secure and not leaking.
Locate the flue gas temperature sensor (if fitted) on the flue outlet and inspect wiring.
Reset the boiler. If F25 recurs, a qualified engineer must check the full flue route and combustion settings.

Warning: Do not use the boiler if you suspect flue problems. Call a professional immediately.

Professional Required 5/5

Flue Gas Temperature Sensor

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