Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
CH Demand
Remove connector X4 from the PCB
Ensure CH controls are calling for heat
Clock switched to ON

Measurement:

Measure the resistance between pin 6 (white) and
pin 15 (white) on connector X4.
Expected resistance short circuit.