Set Appliance State:

Mains Power Off
Set Multimeter to DC Volts Set meter to measure DC Volts
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the DC voltage between pin 3 (red) on
connector X2 and the earth point on the boiler chassis.
Expect 4.6 Vdc or greater.
Note:
Each pin on the PCB connector is numbered.