Short answer
Flame ionisation proves the flame by passing a small AC voltage through it; the flame rectifies it into a DC current measured in microamps. If the current falls below the control's threshold, the boiler assumes there is no flame and locks out.
Common causes
Oxidised or contaminated probe.
Probe out of position in the flame.
Poor burner earth.
Wrong polarity or high neutral-earth voltage.
Unstable flame from poor combustion.
What the engineer checks
Measure ionisation current with a meter in series and compare with the manufacturer's figure.
Check earth paths and polarity.
Repair and prevention
Clean or replace the probe, restore earths and polarity, and correct combustion. Never bypass flame sensing.
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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