Background: I was getting a racing fan (sounds like a leaf-blower) at start-up on my 2002 1.8t. Eventually, it threw a solid CEL, and the code pulled was 16502: Engine Coolant Temperature Cicuit, (G62): High. Due to ECU connectivity problems, my dealer couldn't warranty this repair (for some reason they couldn't connect to my ECU, and it had nothing to do with my REVO.) So...I decided to tackle it myself. I have also heard that this sensor can make the coolant gauge either not work or be very erratic, although I didn't have this problem. I found write-ups that I believe were for the B5...but it seems MUCH harder to get to on the B6, so I decided to do this write-up.

Parts Needed (can get at any Audi or VW dealer):
Temperature Sensor: 059-919-501-A (make sure it's GREEN)
O ring Seal: N-903-168-02 (highly recommend you replace this...mine was very dry and cracked...may want a couple in case you drop one and it lands on the transmission and you can't get it out)
Plastic retaining clip: 032-121-142 (breaks easily...may want a couple just in case)

About $7-$10 in parts:



Tools Needed:
Small flathead screwdriver...I found this the best method for removing the retaining clip.

Beware...this is a serious knuckle banger to get to the sensor and the clip...even if you have average size hands.

Make sure the engine is dead COLD before doing this...or you'll get a coolant surprise right in your face. Remove the coolant cap to remove any back pressure, and then replace. This will minimize to eliminate any coolant coming from the sensor socket.

Here is the location of the coolant temp. sensor...It is at the rear of the block by the firewall...it is somewhat hidden by the wiring harness, but you can just make out the top of the wire clip (inside red circle).



Here is a picture of what you need to do to get to that sensor...I actually had both hands back there. I removed the clip to the fuel line as it was destroying my arm...covered the bare metal bracket in duct tape as it was a tad sharp.



The sensor is held into a vertical tube with a plastic retaining clip. If you feel back there, you can feel the sensor going into the tube, and just below the tube, you can feel a plastic clip sticking out towards the rear of the vehicle. This clip should move back and forth a tad. Now...this clip needs to come straight off, or it will break. I couldn't get my thumbs back there for any grip, so I used a small, flathead screwdriver to pry the clip to it's first "pop", then was able to pull it the rest of the way off.

With the clip removed, pull straight up on the sensor to remove. You may hear a suction type noise when doing this, and don't pull by the wires. The wiring harness is long enough to pull the sensor out into the open so it's easier to work with.



Manipulate the wiring harness clip, and remove the old sensor. Grab the new sensor...and push into the wiring harness until you hear a "click." It can only go on one way. For those needing a picture of this:



Now, you can do one of two things with the O-ring: You can put it on the sensor first, and then put the entire thing into the tube, or you can seat the O-ring into the tube (yup, more knuckle banging) and then push in the sensor. I tried the first way, got the car all together, and then the sensor popped out once the car got hot and spewed coolant everywhere. Here is a pic of the O-ring on the sensor:



Push the sensor into the tube and make sure it's fully seated. If it's not fully seated straight up and down, the clip will not catch the sensor and you'll have a mess when the car gets hot.

With the sensor in, now comes the hard part...The plastic retaining clip has to go straight on. It goes around the tube that the sensor is in, and the inward tabs on the clip go into little slits on the side of the tube...you can actually feel these slits. This clip is what keeps the sensor from coming out, so it the sensor is too high, you'll miss the lip on the sensor that the clip rests on. This may take a few tries and a little frustration, but you'll get it. Once in...give a few test tugs on the sensor (NOT the wires) to make sure it's in there.

Put everything back together...turn on car, let it idle to warm, and road test. Good luck!

DISCLAIMER: This write-up is created for the purpose of benefiting Audi owners everywhere. I take no responsibility in any damage that may incur while attempting the steps in these paragraphs. I am not an Audi technician nor was I supervised by one. Please perform this at your own risk. If unsure or uncertain, please do not perform.