Boiler shows F3 and shuts down, no heating or hot water

Safety: Isolate power to the boiler before removing the case. Do not dismantle the flue or sealed combustion components unless you are a qualified engineer.

• Reset and listen: Reset the boiler. On a demand for heat, listen for the fan starting (a low whirring sound). If there is no sound, the fan may be seized or not powered.
• Check flue terminal: From outside, ensure the flue outlet is clear of obstructions (ice, leaves, plastic bags). A blocked flue can prevent safe fan operation.
• Check condensate pipe: In cold weather, ensure the external condensate pipe is not frozen or blocked. Thaw gently with warm (not boiling) water if frozen.
• Visual inspection: With power off and front cover removed (if user-removable on your model), visually check for obvious loose or damaged cables to the fan.
• Engineer tasks:
  • Test the fan motor for correct resistance and free rotation.
  • Check the fan connector and wiring loom back to the PCB.
  • Verify the PCB is sending the correct voltage to the fan on a call for heat.
  • Inspect and test the air pressure switch or differential sensor if fitted.

Do not attempt to lubricate or dismantle the fan yourself; replacement is usually required if the fan is noisy or seized.