Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X6 from the PCB
(and reconnect plug for gas valve if not already done so)

Measurement:

Test 1:
Measure the resistance between the grey wire marked 4
and the black wire marked 3 on X6 connector.
Resistance should be approximately 2.8kOhms.
Test 2:
Measure the resistance between the red wire marked 2
and the violet wire marked 1 on X6 connector.
Resistance should be approximately 1.6kOhms.
Test 3:
Test between each of the 4 wires (grey, black red and violet)
to a suitable earth point.
Resistance should be greater than 1MOhm on each test.