Boiler shows F3 and locks out, fan does not start or runs erratically
Safety Warning
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
How to Fix / Troubleshooting
Safety first: Turn off power and gas. Working on the flue system and combustion air path must be done carefully to avoid CO risks.
Steps to check:
- Visually inspect the external flue terminal for obstructions (leaves, nests, ice). Clear any blockage.
- Remove the boiler front cover and locate the fan assembly at the top of the combustion chamber.
- Check that the fan spins freely by hand (with power off). If stiff or noisy, the fan may be failing.
- Inspect the silicone tubes to the air pressure switch for splits, kinks, or water ingress. Replace or refit as needed.
- Check the electrical connector to the fan and air pressure switch for secure fit and no corrosion.
- Restore power and reset the boiler. If the fan does not start or F3 reappears, the fan or pressure switch likely needs replacement.
Warning: Do not operate the boiler with the case removed. If you suspect a flue blockage inside the wall or roof, contact a qualified engineer immediately.