Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect CN100 from the control PCB

Measurement:

Check for continuity (zero resistance) of each of the grey wires, in turn, running from the boiler terminal block to connector CN100 from the control PCB.
Check the connectors and harness are in good overall condition.