Furnace shows F2 and shuts down shortly after ignition attempt

Safety warning: Isolate electrical power and gas before removing covers. Flame sensing and combustion checks must be carried out by a qualified technician.

User-level checks:

• Attempt a single reset from the control panel. If F2 reappears, do not continue resetting.
• Ensure air inlets and flue terminal are not blocked by debris, snow, or vegetation.

Technician procedure:

• Inspect and clean the flame sensor electrode (often combined with the ignition electrode on Glow-worm units). Remove oxidation with a fine abrasive pad and wipe clean.
• Check the electrode insulation for cracks and ensure correct positioning in the flame.
• Verify burner and appliance earth continuity; poor earthing can prevent correct flame rectification.
• Measure gas inlet and burner pressure during operation and compare with Glow-worm specifications; adjust or investigate supply issues if low.
• Check the flame signal (µA) using the manufacturer’s test points; if the signal is weak despite correct combustion, suspect the PCB flame detection circuit and replace the control board.