As suggested, disconnecting the red-arrowed one stops the temp gauge working. It's perhaps a bit faulty, as it's reading about 75degC after a good run.
What does the ECU think the temperature is if you unplug the sensor on the stat.
Didn't try that at the time (doh!).
The resistance measurements I'm getting off it seem pretty plausible though. 300 ohms when hot, 4K when lukewarm.
If you unplug the one denoted by the green arrow it should make the cooling fan run continually with the ignition on.
Tried that, and it doesn't come on.
Its a fairly common item to fail, if the thermistor inside has failed it can give a high resistance, and that will make the ECU think the temperature is very cold.
I bet your car also runs rich and uses lots of fuel.[/QUOTE]
I'll need to whip the plugs out and see what they're like. Not had the car long enough to do an MPG calculation.
The ECU warning light only seems to come on after a few miles. If the engine's stone cold, it comes on with the ignition then goes off a few seconds later as it should.
Is there any other sensor that's temperature-sensitive? What about the lambda sensor? Is that the same component as the Heated O2 sensor?