Your boiler is failing to light because it cannot detect a successful flame or is not receiving enough gas to start up.

Start-up failure: Ignit. unsuccessful / Gas meter defective or gas flow monitor has triggered, air in gas, gas flow pressure too low, thermal cut-out has triggered, condensate route blocked, incorrect gas injector, incorrect spare part gas valve assembly, value in D.052 does not correspond to the printed value on the current gas valve assembly, fault on the gas valve assembly, multiple plug on PCB incorrectly plugged in, break in cable harness, ignition system (ignition transformer, ignition cable, ignition plug, ignition electrode) defective, ionisation flow interrupted (cable, electrode), incorrect earthing of product, electronics defective