Set Appliance State:

Mains Power Off
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance.
Does the fan run then the burner ignite and establish?
Or does the boiler return to fault code?
Note:
If the burner ignites and establishes and you believe the fault was not caused by overheat click HERE to continue diagnosis using the full model.