Here is how I replaced the coolant temperature sender on my 2000 VW GTI VR6. I wrote this up because I could not find anything about it on the web. Then I found Arthur's useful post here, but since I have some additional info, I thought I'd post it, even though it repeats much of what Arthur says.
I was getting this DTC on VAGCOM:
17704 - Error in Mapped Cooling System (usually temp sensor or thermostat)
35-10 - - - Intermittent
Also, about one time in ten, when I tried to start a hot (190 degree) engine I had significant difficulty, but eventually got it started after perhaps a minute of cranking. (Usually a second try at starting would be successful.) These episodes were accompanied by a smell of gas.
I planned to change the "G62" temp sensor and went to a VW dealer. They could not tell me which part to get! I bought the "standard" part, 059-919-501A, green plastic for about $6 just as a guess. I also ordered another part on-line (P4036-84658 from VolkswagenPartsStore.com, color blue. The part itself is unmarked, but the box is marked 078-919-501B ) The wiring connector for this part is obviously very different from the 501A part, though the sensor end looks the same as the green one.
I put things off until I got another hard hot start when the car then ran horribly and hobbled home stalling repeatedly. The same 17704 showed, but this time not intermittent.
Replacing the part is NOT easy. It is on the left (driver side) of the engine, pointing forward, a bit in front of the spark coil, surrounded by lots of plumbing and other mysterious parts. Very hard to get at or even see it.
I removed the battery. I removed the left front portion of the plastic manifold cover (one Torx screw.) I also removed one end of a large coolant hose, but that didn't help much because the nipple for the hose was still in the way. (To the outside of the sender unit is a rubber plug in a hole apparently the same size, held in with the same U clip as the sender.) Don't drop anything in this area because it will land on a plastic tray at the bottom and be a pain to retrieve.
I pulled up on the U clip for the sender, and HALF of it pulled out (shaped like a J). Then I noticed that the leg of the clip on the right side of the car is held captive by part of a mounting bracket that would be VERY difficult to remove. It's hard to even see it.
Then luck was with me--I was able to wiggle the sensor out even with half the retaining clip in place. It was still very difficult to disconnect the wiring connector. The old part was colored black and numbered 078-919-501C. I replaced it with the green 501A part. Several places on the web mention that the A part is an improved version of the C part. (The blue "B" part is obviously wrong). I measured the ohms of the two parts, and they were close (Green: 779 ohms A to B, 1749 ohms C to D. Black: 836 ohms A to B, 1700 ohms C to D). I'm not sure the old sensor was even bad.
I was able to get the new sensor back in place, even with the broken clip. If I had to do this again, I would consider LEAVING the old sensor in place and putting the replacement in the hole where the dummy rubber plug is sitting. (This is much easier to reach.) There would still be the difficulty of getting the wiring connector off the old unit though.
Needed about a gallon of coolant to refill.
After all this, I noticed the following DCT in the Instrument section on my VAGCOM:
01039 - ECT Sensor (G2)
30-10 - Open or Short to B+ - Intermittent
where ECT stands for Engine Coolant Temperature. I had not looked under "instrument" before. I suppose this was an old code from before the fix.
Six weeks later, no more DTC codes and no leaks. I guess the old part was bad, even though the resistance seemed OK. Probably intermittent.