Boiler displays F4, burner does not light or goes out shortly after ignition attempt

Critical safety warning: Gas work must only be carried out by a licensed gas technician. Do not attempt to adjust gas valves or burner settings yourself.

Basic user checks:

• Ensure the gas supply to the property is on and that other gas appliances work.
• Check that the boiler’s gas isolation valve is open (handle in line with the pipe).
• Reset the boiler according to the user manual and observe whether you hear the fan and clicking of the igniter.

Technical checks (for qualified personnel):

• Inspect the ignition electrode and flame sensing electrode in the burner area. Look for cracks in the ceramic, heavy soot, or incorrect gap/position relative to the burner.
• Clean the electrodes gently with fine abrasive pad and re‑position according to Protherm specifications.
• Check the ignition cable and ionisation cable for damage or tracking marks. Replace if insulation is compromised.
• Verify that the gas valve receives the correct voltage during ignition. If voltage is present but no gas flow, the gas valve may be stuck or defective and should be replaced with a genuine Protherm gas valve.
• Check the earth connection and polarity of the mains supply; incorrect earthing can prevent flame detection.

After repair: Perform a full combustion analysis and gas rate check to ensure safe operation. Confirm that F4 does not reappear during multiple start cycles.