2001 Monte Carlo LS, 140,000 miles...a while back the coolant temperature sensor started getting flaky...it would read suspiciously low, fluctuate often, and every now and then go from reading to reading nothing in a half second like someone flipped a switch. When the temp sensor isn't reading, the fans are always on, but I also noticed that when the temp sensor isn't working, the AC relay doesn't activate the compressor either. The problem can always be fixed by pulling the + lead on the battery for a few minutes to reset the ECU, but it's gotten progressively worse, and now only works properly for 10 or 15 minutes after the ECU reset, then the temp readout drops and AC compressor doesn't click on at all until you reset the ECU again. I've already replaced the coolant temp sensor before I realized that the AC relay was tied into the issue, and when the temp sensor is working properly the temp readout displays a good coolant temp, thermostat is working OK, etc.

Is there an ECM that controls both the readout from temp gauge and the AC compressor relay? Any other advice?

 

Yes.
When your pcm senses an issue with the cooling system, it goes into a failsafe mode that keeps the fans turned on and disengages the a/c clutch so that the a/c compressor cannot be turned on.

Have you checked all of the wiring and connectors leading up to the coolant temp sensor?
What makes you sure that the thermostat is working ok?
Have you scanned the car at all for dtc's?

 

How would a problem with wiring and connectors be (albiet temporarily) fixed by resetting the ECU, and NOT by just shutting the car off for the same amount of time?

The thermostat is OK because when you reset the ECU, while the temperature sensor is reading, the car heats up fully to temp but never overheats, even when sitting in traffic.

The car has DTC's, but from other issues, because it's a pile. lol The last time I had it scanned (which was while this issue was occuring) the only DTC's that read out were O2 sensor over threshold because I replaced the cat with a straight pipe after the third cat fouled with an essentially brand new fuel and ignition system since I had spent over a grand replacing every last bit that could have been causing poor ignition and premature cat fouling. Also, the time before that it had the O2 error and an error for a bad misfire after the second brand new ICM went bad on me. This thing has seriously been nothing but trouble since the day I bought it four years ago. Paid it off this month, thank god. lol

 

Because resetting the pcm by unhooking the power supply completely resets its memory. That does not happen when you restart the car. When you restart the car, it still thinks it is in failsafe mode until it the parameters that caused the failsafe to set come back within their set limits.
When you do a complete reset, it is seeing something out of the set parameters that is throwing it off however minutes later, but until then it thinks it is fine.

Either way, I really don't think it's the wiring.

I'm almost positive that your t-stat is bad.
When they fail, they stick open.
So you'll see it go up to temp, but a little cooler than it's supposed to be (depending on outside temp).
The PCM realizes that the t-stat is failing, and goes into failsafe, causing all of your issues.


Also, did you get the misfire rectified?

 

By 'up to temp' I mean fully up to temp. It hovers between just a hair below the middle line and just above the middle line in 30-40 degree weather, like it has since I bought it...then randomly starts dropping even if you're not moving, then just drops like you flipped a switch and won't read at all. When the PCM is in failsafe mode does it disable the temperature readout on the gauge? After the PCM goes into failsafe, it NEVER lifts at all, not even a smidge, even if the car is fully up to temp...I can turn it on and let it sit in the drive way for 30 minutes and the gauge won't budge...I'll shut it off and pull the battery for 10 minutes, plug it back in and start it back up and the needle jumps up to normal operating temp the second the key goes to 'on'.

And yeah, the misfire was the ICM. Replacing it both times fixed the problem, but the ICM went bad the first time I replaced it like 3 months later.

 

thermostat bad I had this on my 2001 impala ,I replaced thermostat that fixed my car.

 

Yes. This happened exactly as you describe on my old 2001 Monte LS.
Well, almost. It was random on mine. It would run fine for a while, then randomly the temp gauge would shut off and the fans would stay on and I'd lose a/c.
Replaced the t-stat, everything went back to normal.
There's a chance that your issue is being caused by something else, but either way the pcm is going into failsafe mode for the coolant. All of the issues you are seeing are caused by that failsafe being tripped.

 

I keep having this same problem. Works fine all summer long and then the cold weather comes and it starts acting up. In my case the thermostat has recently been replaced, the temp sensor 3 times. If I disconnect the battery AND UNPLUG the sensor connections and leave it for 2 hours, SOMETIMES it corrects it for a day or 2. I liked cars better before they got all high tech. I have had it to the dealer to check the wiring for the sensor, they say it's all good. This particular vehicle of mine is a very temperamental lil BITCH! Guess I got the right car to match my personality. lol

 

This thread is quite old, what issues are you having?