Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove electrical connector from gas valve

Measurement:

Measure for resistance across pins 4 & 5 of the gas valve terminals.
Expected resistance approximately 8.6 - 8.9 Mohms.
Test 2. Measure for resistance between pin 4 & a suitable earth connection.
Expected resistance 1 Mohm or greater.