Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB

Observation:

Inspect the low voltage harness (large molex plug on RHS of PCB marked X20) for any signs of damage or wear to the insulation that would allow it to short to earth.
Pay particular attention to any place where
the wires are bent or are near the boiler
chassis / heat exchanger.