Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the 4 way connector from the PCB

Measurement:

Test 1.
Measure the resistance between pin 1 (brown) and
pin 2 (blue) on the 4 way connector.
Test 2.
Measure the resistance between pin 1 (brown) on 4 way
connector and a suitable earth connection.
Test 3.
Measure the resistance between pin 2 (blue) on 4 way
connector and a suitable earth connection.
Expected resistance greater than 1 MOhm on all tests.
Note.
A reading of O.L on each test is acceptable as this
is greater than 1 MOhm.