Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
Selector Switch to Hot Water and Heating
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run.
Measure voltage between Black (6) on connector X401 and Earth.
Expect approximately 6VDC
Is the voltage as expected?