First off - 1998 A6 Quattro, 2.8l 30v

I removed the existing temp sensor - blue square sensor. I replaced it with a green square sensor. When I started the car the temp sensor light came on. Do I need to disconnect the battery to reset the sensor?

I thought the green sensor was round and the blue one was square? I definitely replaced a blue square sensor with a green square sensor.

 

check the electrical connector, they often get tarnished and when you pull them apart the female spread a little so pinch them back together and clean them with a nail file, apply a little dielectric grease and you should be better. If your light is on it should be because your not getting a reading from the sending unit at all.

 

tried all that you suggested and still no go. the car runs for about 30 seconds and the temp light comes on

it worked before?? I was having cold start issues this past winter and I have warm start issues if I drive the car for a while, stop, and then restart within a few minutes. A guy that's in the Guard with me hooked it up and the error code was related to the temp sensor (that's why I was replacing)...

do I need to try to replace the connector (gonna be a pain in the butt for sure) or what else can I do?

 

It is possible that the new sensor is bad, happens. You might try testing it or getting another one if you are feeling lucky. I replaced one on my Volvo and it worked for two weeks and quit. Another one worked okay.

 

everything's working fine now. I took it apart again yesterday, blew it out with compressed air, made sure the ends were squeezed together good, and applied more dielectric grease...cel has gone off and no other lights lit up (currently)