Set Appliance State:

CH Demand
CH temperature control to max
ON/OFF switch ON
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Confirm appliance is cool, if not allow appliance to cool.
With the fault code displayed does the appliance reset ?
Press and hold the reset button for 5 seconds.
Boiler resets - Further investigation overheat
Boiler resets but returns to fault code