Statey - I sold my rsturbo in about 2000 or something so it has been a while!

However I spent LOTS of time working on it and you don't tend to forget!

It will tell the ECU the temperature of the water in the engine. From this, the ecu decides if the car is cold or hot. If it thinks it is cold, it adds lots of fuel, kinda like a choke did on an old motor.

So basically the thermostat "thing" provides a different resistance when cold to hot through an entire range of movement/values. When you don't plug it in, it assumes the car is utterly freezing as the resistance is really high and therefore adds tonnes of fuel.

That is all correct in theory, however, I didn't ever check the resistace so the scale might be opposite to how I have mentioned. IE higher resistance when hot than cold, but at the end of the day, the theory is the same and if that plug is not connected properly, the car will decide to add tonnes of fuel

Good luck sorting out the wiring, check all the plug colours match their connector colours too

Cheers

RW