Set Appliance State:

Ensure boiler is cool
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Press the reset button on the display (rightmost button).
Does the boiler reset and enter standby or is the fault still shown?
BOILER RESETS AND ENTERS STANDBY
(test CH operation through full model
BOILER DOES NOT ENTER STANDBY - FAULT STILL SHOWN
BOILER RESETS BUT DIFFERENT FAULT CODE SHOWN