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
Use Insulation Mat as required
Gain access to the PCB
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Once the boiler fires for a CH demand (this may take up to 3 minutes) measure the voltage between pins 1 (Blue) and 3 (Black) of plug X13.
Expect mains voltage.
Note:
The colour of the wires may vary as there are different harnesses
through the boiler production but test points will remain the same.