Short answer
The ignition electrode produces the spark that lights the gas. Over time the tips erode, the gap widens, ceramic insulators crack and deposits build up — the spark becomes weak or tracks to earth instead of crossing the gap.
Common causes
Eroded tips and widened gap.
Cracked ceramic insulator causing spark tracking.
Deposits from poor combustion.
Damaged HT lead or connector.
What the engineer checks
Inspect electrode condition, gap and position relative to the burner.
Check the HT lead and ignition generator output.
Repair and prevention
Replace the electrode and its gasket to the exact gap specified. Combined ignition/ionisation electrodes affect flame sensing too, so check the flame signal after fitting.
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.
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Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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