The coolant sensor switch has been known to degrade slowly - it sends inaccurate signals to the ECM. The best part is that the R&R takes no expertise and it's what I call a 5 n' 5. It costs approximately $5 and it takes approximately 5 minutes to replace!!
The tech article on AW is a little out of date (and it is written, with diagrams, for a 99 A4). The sensor is found at different locations on the 1.8t and 2.7t engines; determine where your sensor is located before proceeding with the following instructions.
It's truly a very easy Remove & Replace, but I'm going to go into every detail for the sake of safety. I will emphasize right now that there are two parts that you could damage or lose because they are small. They are the retaining clip and the O-ring that seals the sensor inside the sensor's housing. If you go slowly and use your head, you won't have any problems with either of these parts.
YOU NEED TO DO THIS R&R WHEN THE ENGINE IS COOL.
First, you need to remove the wiring harness from the sensor. There is a pressure clip (that will release the harness) that is designed and operates just like many of the other wiring harnesses throughout the engine bay.
NOTE: Some people like to leave the harness on so they can use the harness and wire set to pull the entire unit out. Then they unclip the harness from the sensor once it's out. That's OK if you think you won't get leverage, BUT... when you pull on the unit, make sure you are NOT pulling on the wires only or you may damage the wires. (It's like when you're home vacuuming and then you yank on the vacuum cleaner's electrical cord, rather than walking over to the outlet and pulling on the plug. WRONG!)
Next, you need to remove the PLASTIC horseshoe shaped retaining clip that secures the sensor in its housing. Pull it straight out - NOT UP, but parallel to the ground. It's plastic, so if you don't pull straight out, you might snap it - not good. It will have a little tension. Just pull firmly and it will give. PUT THE CLIP SOMEWHERE SAFE. It's too small and to vital to lose.
Now you need to pull the sensor out.
NOTE: The sensor is sealed in its housing by a small rubber o-ring. It may come up with the sensor or it may remain inside the housing. Regardless, don't lose it! Some people replace it with a new one while most re-use the old one because the o-rings rarely have damage.
Now just pull the sensor straight up and out. It will resist due to the o-ring, but it will suddenly come loose. Just get a good grip and pull firmly. At this point you may get splashed by a little coolant - don't worry about the coolant (unless you didn't heed my caution to do this on a cool engine!) - it only gets on your hand.
Now all you do is reverse the process for the installation of the new sensor. No need to replace the spilled coolant - it's too little to worry about.
NOTE: If the sensor is not seated all the way into the housing, then you won't be able to replace the retaining clip properly - it won't snap into place. If there is a problem, it's probably the o-ring. Check to see that the o-ring isn't kinking around the sensor.
You're done and have saved yourself hundreds of potential repair dollars in just five minutes!!