Audi Repair: Temperature gauge and warm Aircon, coolant temperature sensor, poor fuel economy

Question
For some reason my audi A4 B6 ALT's temperature gauge is no longer rising to 90 even when I have driven for more than an hour. Since this started my aircon no longer heats up even when temp is set on HI, it just warms up. The only time when my gauge points at 90 is when I drive in heavy traffic but still my aircon won't heat up.

Answer
It sounds like your thermostat is stuck open, and most of the heat from your engine is going into the radiator, not the heater.  You should also have noticed a poor fuel economy.  I would check/change the thermostat first, and then go from there.  The coolant temperature sensor is a high failure item, so it may be worth chacking that too, but since you have the other symptoms, I'd go for the thermostat first.  Let me knwo what you find,  Jan