Oh you already changed the sensor? Well then it's the thermostat. You start by changing that sensor and if that doesn't fix the problem next step in the stat.

 

I am having a similar issue with a 2001 1.8t. Where is the sensor located? Does anyone have a picture?

 

Here is an excellent, easy to use guide mate:

http://www.audiworld.com/tech/eng47.shtml

There are two colours of sensor so make sure you get the right one for your car. There are a green and blue one and can be picked up for around a fiver and £12 repectively on ebay.

The chances are the problem is the thermostat but it will do no harm to rule this our first and as I say it only costs a few pound and takes 10 mins.

You will loose coolant but if you're quick enough you won't lose that much. I avoid putting water in the coolant tank and always use 50/50 coolant but what I did was top the tank up with water (after losing some changing the sensor) and took the car out for a spin to see if that solved the problem as either way I was going to change the coolant anyway. When I noticed that it didn't help I went and bought coolant and ordered the stat from ebay, changed the stat and flushed the system and all is well now.

It is very easy to do so if the sensor doesn't work for you let me know and I'll link the guide for changing the stat.

Here are links to the sensors on ebay: The blue one is £12 and is what I needed to for a 98 1.8T

Green
http://cgi.ebay.co.uk/AUDI-COOLANT-T...item27b45c0d88

Blue
http://cgi.ebay.co.uk/AUDI-COOLANT-T...item27b45c0d8a

Here is the link for the thermostat:
http://cgi.ebay.co.uk/AUDI-A4-A6-1-9...item4cf4fd064a

 

well i have changed thermostat, water pump, and ect sensor.. maybe the upgraded intercooler in the front is just preventing alot of airflow and therefore the fans behind the intercooler are only able to push hot air onto the engine.blahh