Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
CH Demand
DHW Selector in Minimum (-) position
CH Selector in Maximum (+) position
ON/OFF Switch in ON position
Boiler temperature must be at MAX for this test (75°C)
If necessary run CH till MAX temp reached
Disconnect connector X20 from the on the PCB

Measurement:

Measure resistance across connections 13 (black) and 12 (black) at the disconnected connector X20.
Expected resistance closed circuit zero resistance.