Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Boiler ON/OFF Switch OFF
CH Temperature Selector OFF
DHW Temperature Selector OFF
Remove lower front facia panel

Measurement:

Measure resistance across terminals 1 & 2 on connector ST13 on the main PCB.
Note: A link should be present between these two terminals.