Boiler displays F4 and will not fire, intermittent ignition then lockout

Safety: Turn off power and isolate gas before any internal work. Gas pressure and combustion checks must be done by a Gas Safe engineer.

• Reset and observe: Reset the boiler and watch the sequence: fan runs, ignition clicks, brief flame then shutdown with F4 suggests unstable flame.
• Check gas supply: Confirm other gas appliances operate normally. Ensure the boiler's gas isolation valve is fully open.
• Ventilation and flue: Ensure air vents (if any) are not blocked and the flue terminal is clear.
• Engineer tasks:
  • Measure inlet and burner gas pressure against Glow-worm specifications.
  • Test the gas valve for correct operation and coil resistance.
  • Inspect and clean the burner and injector for debris or corrosion.
  • Check ignition electrode position and flame sensing continuity back to the PCB.

If F4 recurs, do not continue resetting repeatedly; arrange professional service to avoid unsafe combustion.