Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Remove Water Ingres 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 fault code?