Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
DHW Selector in Minimum (-) position
CH Selector in Maximum (+) position
ON/OFF Switch in ON position
Disconnect the internal programmer to the right of the display board
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler now respond?
Note:
If there is no internal programmer fitted, this test should be skipped (select boiler does not respond)