Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Central Heating Switch in constant position
Boiler Switch in ON position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Is mains voltage present between connector PL3 connection 3
(red wire) and connector PL1 neutral connection 8 (blue wire)?
Note:
Boiler may go to lockout.
Switch OFF mains power and press reset button to cancel lockout.