Set Appliance State:

Mains Power Off
Ensure appliance is cool (below 40°C)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset the appliance (turn selector switch to the reset position hold for 2 seconds then release)
Turn on the hot water tap and allow the boiler to commence its ignition sequence.
Does the burner ignite or does the appliance return to the overheat fault code after attempting to ignite?
Note:
If the burner does ignite and establish once confirmed turn the demand off.