Short answer
A flame failure is the control box seeing the flame signal disappear, or never appear, when it expects a flame. It shuts the gas valve to prevent unburnt gas building up in the combustion chamber. The cause is either a genuine loss of flame or a loss of the signal that proves it.
Common causes
Weak flame signal from a contaminated, oxidised or poorly positioned ionisation probe.
Gas pressure dropping under load, starving the burner.
Incorrect combustion setting, giving a lifting or unstable flame.
Flue gas recirculation or blocked flue disturbing the flame.
Poor neutral-earth voltage or reversed polarity affecting rectification-based flame sensing.
What the engineer checks
Measure the ionisation current at low and high fire and compare with the manufacturer's minimum.
Check gas inlet pressure at full fire, with all boilers in the cascade running.
Run a combustion analysis at minimum and maximum rate.
Check polarity and neutral-earth voltage at the boiler supply.
Repair and prevention
Clean or replace the probe, correct the gas/air setting, fix supply pressure or flue faults. Do not repeatedly reset a boiler with flame failures — find the cause.
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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