Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Selector Switch ON
Gas Control Knob in Main Burner position
Disconnect connector CN4

Measurement:

Measure resistance across CH Thermistor (Orange wires).
This should be in the range 2K ohms (Hot) to 13K ohms (Cold).