Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Unplug connector CN4

Measurement:

Measure resistance across CH Thermistor (Orange wires).
This should be in the range 2K Ohms (Hot) to 13K Ohms (Cold).
Replace connector CN4.
Note:
This test should be carried out on the disconnected connector not on the PCB.