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
Remove the electrical connector from the gas valve

Measurement:

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