Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector T2 from the main PCB

Measurement:

Measure the resistance across the rectification section of the boilers main harness between T2 and the inline connector disconnected on the previous test.
Expect continuity.
Also visually inspect the harness for general condition and any signs of damaged insulation.