I own a 2003 Passat Wagon with a 1.8T AWM type engine. Here is my story on code P2181 and replacing the coolant sensor. I don't know that this solves P2181, but it is what I have done so far. This is going to be a long post so I ask if you're going to quote it please crop what you are going to specifically quote and not quote this entire post. Thank you.
A while ago my check engine light came on and when I checked with my reader it had code P2181. I googled the code and found that most people on forums suggested replacing one of several coolant related things -- most often the coolant sensor -- to rid of that code.
Eventually I found this insight on newbeetle.org from a VW tech:
Over the last few months I erased the code continually. Each time it eventually would reappear (and oddly enough sometimes disappear) on its own. A little more research and I found out that it would probably be worthwhile to check the freeze frame data to help isolate the problem part. Unfortunately I don't have a reader that can retrieve freeze frame data (I have a CodeKey). And so I decided to replace the cheapest (and what I thought to be the easiest!) part first, the coolant sensor.Originally Posted by agental@http://newbeetle.org/forums/2-0-liter-gas/5464-recurring-code-p2181-cooling-system-performance-2.html
I had previously replaced the coolant sensor on a Hyundai and I've found VW was different because from what I'd read VW's coolant sensor is also a coolant sender. The Hyundai I worked on had a coolant sensor (to signal to the computer to turn the fans on) and a separate coolant sender (to signal to the dashboard gauge the temperature). VW's coolant sensor/sender is essentially a dual element in one unit and either element can fail independently of the other. This is explained in a VWvortex post, but unfortunately their archived site data is offline as of June 8 2010.
I went to VW dealership and bought a coolant sensor and an O-ring. I gave them my VIN rather than part numbers to get the right parts. Looking back it was a huge mistake was not buying a few (or even one) of the cheap coolant sensor retaining clips. I did not realize how hard it would be to remove the retaining clip without breaking or losing it.
I found a great thread here that describes in detail with pics how to change the coolant sensor:
Coolant sensor replacement on 1.8T AWM - with pics - Passat World Forums
I followed the instructions in that thread and but it was very hard for me and I could not do it exactly as instructed. I should note I did not take the heatshield off. It didn't seem to be a problem. I should also note I have skinny arms and hands. If my arms or fingers were even a little bit thicker I don't know if I could have done it in any way.
Here is how I deviated.
First I needed a very specific type of screwdriver, a skinny flathead with a (>=)6" shank. I had other screwdrivers just as long but their shank was too thick or too short to work in such a tight space.
Second I could not for the life of me pull out the retaining clip in the way instructed. I tried really hard but there was no getting around metal tubing that was in the way. Here's a picture to get an idea of things:
Clip.jpg (alternate link)
So I tried really hard to go with the screwdriver through that hole above, for a long time with a lot of patience but it wasn't happening. I could pop the clip out a tiny bit but not enough. I decided to reposition my screwdriver from the right side instead. Looking through the hole as seen in the pic above I was able to guide the screwdriver in from the right side and work on the clip. Here's a picture to get an idea of things:
Screwdriver.jpg (alternate link)
So at this point I was so frustrated I decided I was just going to bend the clip and even if it breaks I didn't care I would just have to wait until Monday and buy another one. So using my screwdriver from the position in the pic above, and with my left hand reached in (as described in step #2 from the directions) and feeling the clip, I was able not to slide it out, but rather to pop the right side leg out of its groove. It snapped back in a few times but repeating this process, and working and attempting to rotate the clip in its compromised position with my hand, I was able to get the clip to pop out. And luckily I was able to very patiently work it into my two fingers and take it out!
The rest of the procedure was kind of difficult but nothing compared to removing the clip. When I removed the old sensor the old O-ring did not come with it, so I did what it said in the directions and stuck my thumb in the hole and coaxed it out. Nearly impossible without dainty hands.
Replacing the sensor was a difficult because I tried a few times to get it back on and almost lost the new O-ring in the process. In hindsight a few more O-rings in case they fell somewhere would have been a good idea. It was hard to sit the new sensor as it should sit and even when I had it sitting right I wasn't sure if it was all the way in. But after careful deliberation of the before and after feel (make careful note of the before feel and orientation) I decided it was and proceeded to put the clip back on.
Naturally I did not put the clip on in the same way I took it off. The major problem was that the clip was previously oriented towards the engine. Instead when I put it back on I made sure the horseshoe part faced the opposite way (towards the cabin). It was hard to get the legs to line up with the grooves initially and again I almost lost the clip a few times. But eventually it went on. Someone suggested tying dental floss around the clip so you don't lose it; that might work. I live on the edge and I didn't.
Total time from prep to cleanup was several hours for me. I still don't know if this will solve P2181 but I'll make posts to this thread with any additional info. Hope this helps any of you out there in a similar situation.
P.P.S. Also here's a general pic from taligentx of what a retaining clip looks like on a VW passat. They show metal but in my situation it was plastic. But if you want an idea click here to see their pic.
P.P.P.S. Another interesting thread on P2181 can be found here.