Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Selector Switch in Winter (II) position.
Remove connector J13 (bottom right) from PCB.
Mains Power ON

Measurement:

Measure resistance across black wires of
connector J13 (not PCB)
Mains Power OFF
Refit connector J13