Set Appliance State:

Mains Power Off
CH Demand
Set the boiler control switch to O position
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the boiler to self check.
If the boiler shows any character other than '0' press the reset button and allow boiler to reset.
Does the display show '0', '1', '2', or 'h'?