Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to AC Volts Set meter to measure AC Volts
Turn Hot Water Tap On
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:

Allow the boiler to fire for DHW.
Measure the AC voltage between pins 3 (blue) and 5 (black) on connector CN5.
Expect mains voltage.
Is the voltage as expected?