On my way back from a long trip my check engine light came on and the temp. gauge stopped working, it would move from hot to cold. Well not too long after i made it home the light went off and the gauge started working again so I ignored it. Then not too long ago it started doing the same thing. Today I replaced the sensor and it still is doing it. Any suggestions what this could be? The wires seem to be fine. When changing it a little coolant did leak out on the part that connects to the sensor, could that be throwing it off?? Thanks.

I just got a reading and it is coming up engine coolant temperature circuit high input if that helps

I believe that the output voltage of the ECT sensor is directly related to the coolant temperature, thus, a "high input" code would essentially translate into a high coolant temperature, typical of an overheating scenario. This could be plausible for the first scenario in which you had just engaged in a lengthy trip, however, you have not described encountering an overheating issue. In addition, you stated that the gauge was "temperamental". Adding the lack of overheating to the possible gauge malfunction, I would do as you have done and lean towards an errant sensor.

As far as getting coolant on the sensor, I don't foresee this as being an issue (though I would not stake my life on it), as when we do coolant flushes we routinely douse these with coolant, and if a coolant-based sensor is not designed to come in contact with coolant, I would have to chalk that up as horribly poor design.

I guess this somewhat narrows it down to the wiring if the sensor has been replaced. You say the wiring appears fine, though the wire in it's entirety must travel from the sensor to the PCM, so there may be a short hidden somewhere under the loom.

That's my 2 cents...

thanks for the help, I sorry i didn't mention it, but i have not had any overheating problems. I have noticed one change since Ive changed the sensor though, now the gauge does not fluctuate like it was, but just stays cold. Actually when I start the car it goes to the left (98 cavalier btw) about 1/16" - 1/8". I haven't had a chance to check all the wiring, as I was trying to avoid it, but tomorrow im going to drain the fluid take the sensor out try cleaning it off then replacing it and refilling it. If that don't work then im going to have to check all the wiring. =/. Until then any other advice will be greatly appreciated!! Thank you!!