Set Appliance State:

Mains Power Off
CH Demand
Ensure appliance is cool
Boiler thermostat to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the ignition sequence to commence.
Does the burner ignite or does the overheat LED illuminate before ignition has taken place?
Note:
If the burner ignites, investigation into the cause of the overheat situation will be carried out.