Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove connector CN1 from the DBI PCB.
Remove connector from gas valve.

Measurement:

Measure the resistance of each of the wires in the harness running between CN1 and the connector from gas valve.
Expect continuity / zero resistance
Also visually inspect the condition of the harness.