Short answer
Ignition failure means the boiler ran its start sequence — pre-purge, spark, gas valve open — but never established a proven flame within the safety time, so it locked out. Every ignition failure comes down to one of three things: no spark, no (or wrong) gas, or a flame that formed but was not detected.
Common causes
No spark: worn, cracked or wrongly gapped ignition electrode, failed ignition transformer or generator, damaged HT lead or a poor earth.
No gas: closed isolation valve, low inlet pressure, gas valve not opening, blocked gas filter, or air in a newly purged supply.
Flame not proven: dirty or misaligned ionisation probe, poor burner earth, or incorrect gas/air ratio producing an unstable flame.
Air problems: blocked air intake or flue causing the fan to fail to reach ignition speed.
What the engineer checks
Watch the start sequence through the sight glass and listen for spark and gas valve operation.
Measure gas inlet working pressure during the ignition attempt, not just standing pressure.
Check electrode gap and condition against the manufacturer's dimensions, and the flame signal (µA) after ignition.
Confirm earth continuity between burner, PCB and the electrical supply.
Repair and prevention
Replace electrodes and gaskets as a set at the service interval, set gaps with a gauge, and restore earths. If gas pressure collapses during ignition, investigate the supply rather than adjusting the boiler.
Record the lockout history — a boiler that fails ignition only when other plant is firing usually has a supply pressure problem.
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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