Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
CH Demand
Gain access to PCB (use Insulation Mat as required)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

If required, reset boiler.
Measure for A/C voltage across connector 20 brown & blue leads.
Expected voltage, mains voltage.
Note.
1. PCB connector numbers can be found on the board.
2. Ensure a good connection is made or misdiagnosis will occur.