Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector X20 from the PCB

Measurement:

Test 1:
Measure the resistance between pin 9 (Blue)
and pin 18 (Red) on connector X20.
Expect approximately 45 ohms.
Test 2:
Measure the resistance between pin 18 (red)
and the earth point on the boiler chassis.
Expected resistance greater than 1 MOhm.
Note:
Each pin on the PCB connector is numbered.