Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
CH Demand
WARNING! - Mains voltage is expected during test follow TOPs P4 live testing procedure
Mains Power On
Set CH Stat to MAX
Allow boiler to run on CH and allow to reach temperature and burner to shut off
NOTE:The system may reach temperature quicker if you reduce the load by shutting off some of the radiators
Isolate Mains Power to appliance as per TOP's P4
Disconnect connector from X4 on the PCB

Measurement:

Measure resistance across black connections 1 & 2 (black wires) on the disconnected connector.
Expected resistance closed circuit zero resistance.