Ideal boiler displays F5 and locks out, often accompanied by loud ignition or poor burning

Safety first: Flame detection and combustion issues are safety critical. Only a Gas Safe registered engineer should adjust combustion or access the burner.

User checks:

• Reset once: Turn the boiler off and on, then press reset. Observe if the boiler runs normally or quickly returns to F5.
• Check for signs of poor combustion: Yellow, sooty flames visible through a viewing window (if present), or soot around the flue terminal, indicate a serious issue. Turn the boiler off and call an engineer immediately.

Engineer actions (for information):

• Clean or replace the flame sensor (ionisation electrode).
• Check earth continuity and wiring to the electrode.
• Perform combustion analysis and adjust or replace the gas valve if required.
• Test PCB flame detection circuitry.

Do not continue to reset the boiler if F5 recurs. Arrange professional service.