Boiler displays F9 and locks out, no heating or hot water

Safety: Switch off power and isolate gas before internal inspection. Only a qualified engineer should remove the combustion cover.

• Reset and monitor: Reset the boiler and observe if the burner lights then goes out quickly. This suggests the flame is not being correctly detected.
• Check earth and supply: Ensure the boiler is properly earthed and that the mains plug or spur is secure. Poor earthing can affect flame sensing.
• Engineer tasks:
  • Remove and clean the flame sensing electrode with fine abrasive, ensuring correct positioning.
  • Check the flame sensor lead for damage and secure connection to the PCB.
  • Verify gas pressure stability and combustion quality.
  • Test the PCB flame detection circuit if all else is correct.

Persistent F9 faults require professional diagnosis to ensure safe combustion.