Thanks I think your right ... It is the one that is stuffed in the mess at the back of the motor (ETS)

Did you know that there are 4 temp sensors on an Audi?
The CTS -coolant temp sensor at the bottom of the radiator,
ECT engine temp sensor at the back of the engine (Motor),
ITS interior temperature sensor on the dashboard,
and OTS outside temp sensor someplace near the front of the vehicle

 

Yes he is referring to the one behind the passenger cylinder head in the coolant crossover pipe. And for future reference, the outside temp sensor is mounted on the lower crossmember in front of the radiator, center of the car.

 

In my C4 there's one outside temp sensor mounted on the front bumper's inner suport, and also there's another one mounted under the cowl, which 1 yr ago no one knew to tell me what it is.

 

The radiator fan temperature switch is on the bottom of the radiator on the C4 chassis 1996 A6. The switch controls the high and low speed radiator fan relays. The third pin of the switch is grounded.