Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Turn Hot Water Tap On
Set flow rate at tap to approximately 7 ltr/min
Gas On
Mains Power On

Measurement:

Allow boiler to fire and heat DHW to approximately 55 degrees C.
Turn off hot tap.
Mains power OFF.
Disconnect plug from DHW temperature sensor (orange wires).
Measure resistance across the exposed terminals of the DHW temperature sensor.
Replace plug after test.