Set Appliance State:

CH Demand
Isolate Mains Power to appliance as per TOPs P4
Remove water ingress from impulse tube
Fit impulse tube back to gas valve and combustion chamber
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Reset boiler if necessary.
Allow ignition sequence to commence.
Does boiler light and establish now or does it revert
back to the lockout?