Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Disconnect LV1 from the PCB

Measurement:

Test 1:
Measure the resistance between pin 1 (red) and pin 2 (black) at connector LV1.
Resistance depends on which gas valve is fitted:
Honeywell valve: Resistance should be approximately 115 Ohms.
SIT valve: Resistance should be approximately 70 Ohms.
Test 2:
Measure the resistance from pin 1 (red) to a suitable earth point.
Resistance should be greater than 1M Ohm (or O.L on meter).
Is the resistance measured as expected?
Note:
Connector may differ slightly in appearance but this does not affect the test.