1997 audi A4. The heat does not work very well. When cruising on the highway the temp guage drops all the way down. When idling it goes back up. Could this be just the thermostat is stuck open? Someone told me its the coolant temperature sensor. Any comments?

i would have to go with the thermostat only because the heat doesn't work that well you said, if it was the sensor the heat would work normal, and the gauge would be bouncing around, thermostat would be the cheepest thing to check and replace.

Yeah! I also think it is cooler temperature sensor.