Short answer
If the burner lights but the boiler locks out a few seconds later, ignition is working but flame proving is not. The control box opened the gas valve, the flame formed, but by the end of the safety time it did not see a strong enough flame signal — so it shut down.
Common causes
Low ionisation current from a dirty, cracked or wrongly positioned flame sensor.
Poor burner or PCB earth, or incorrect polarity.
Gas pressure collapsing as the burner ramps up.
Gas/air ratio too lean or rich for a stable flame at ignition rate.
Condensate backing up into the combustion chamber.
What the engineer checks
Read the flame signal live and observe the flame at the point of lockout.
Measure gas inlet pressure as the burner ramps.
Check earth continuity, polarity and the condensate trap.
Repair and prevention
Restore the flame signal (probe, earth, polarity), correct combustion settings, and clear condensate. Repeat the ignition cycle several times to confirm a stable result.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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