Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect gas valve connector
plug from the PCB

Measurement:

Test 1: Measure resistance across the red and black leads of the disconnected plug.
Expected resistance approximately 4200 Ohms.
Test 2: Measure resistance from the black lead on the disconnected plug to earth.
Expected resistance open circuit.
Test 3: Measure resistance from the red lead on the disconnected plug to earth.
Expected resistance open circuit.