How to Fix / Troubleshooting

Safety: Turn off power and gas. Do not operate the boiler with suspected flue or fan faults.

User checks:

Visually inspect the external Keston flue terminal for obstructions (leaves, nests, snow, plastic bags). Do not insert objects into the flue.
Check that the condensate pipe is not frozen or blocked, especially in cold weather.

Engineer checks:

Remove the boiler front cover and verify the fan spins freely by hand and is not noisy or seized.
Check the fan receives the correct voltage from the PCB during start-up. If powered but not running, replace the fan assembly.
Inspect the air pressure switch tubing for splits, kinks, or water ingress; clear or replace tubing as needed.
Test the air pressure switch contacts with a multimeter while the fan is running; replace the switch if it fails to change state.
Check and clean the condensate trap and internal flue connections for blockages.

Boiler displays E04 and will not fire, fan may not start

Possible Causes