Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the silencer for better access to the exhaust temperature sensor
Remove the connector from the exhaust temperature sensor

Measurement:

Test 1.
Measure the resistance between pin 6 (brown) on connector X3 to
the corresponding wire on the exhaust temperature sensor connector.
Test 2.
Measure the resistance between pin 18 (blue) on connector X3 to
the corresponding wire on the exhaust temperature sensor connector.
Expect continuity / zero resistance on each test.
Also visually inspect the condition of the harness and connectors.