Your boiler is failing to light properly or cannot detect that a flame has started, resulting in a safety shutdown to prevent unburnt gas from building up.

Technical description

No flame or no ionisation (after 3 starts). a. no ignition spark (check connection of ignition cable, spark plug cap, electrode for breakdown, and distance 3-4 mm). b. ignition spark, but no flame (check gas cock, gas pressure, gas pipe venting, gas valve, and gas/air ratio). c. flame, but no ionisation.