Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect the 4 pin connector with yellow leads from the PCB

Measurement:

Measure the resistance between connection 1 at the
disconnected connector to a suitable earth point.
Repeat test on connection 4.
Resistance should be greater than 1M Ohm (or O.L) for both leads.
Inspect harness is in good condition throughout, pay particular attention to anywhere it is near the boiler chassis.