The 'thermister' is physically located on the V coolant pipe under the inlet manifold.

To test it, just remove the white electrical connection from the idle ecu that has 4 wires on it.

Set your multi meter to resistance (0-100 k ohm) and connect the multi meter probes to the Black/Slate (grey colour) and the Black/Yellow cables (not the ecu itself)

You should get approx 12k ohm at engine warm temperature.

If you get open circuit then that's your problem. I.e. a wiring problem or connection problem.

Good luck

Nick