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 13 (Red) on connector X2 and the earth point on the boiler chassis.
Expect approximately 5 Vdc.
Note:
Each pin on the PCB connector is numbered.