Set Appliance State:

Mains Power Off
Ensure no demands
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the display operate?
If the display shows OFF on its left press the mode button until the radiator symbol shows.
With no demands boiler should now be in standby mode:

No fault code should be displayed and the boiler should not attempt to fire.