I have a 2000 Monte Carlo SS that when driving temp gauge will be at normal 190 but intermittently gauge will start to fall and go all the way to lowest possible reading of 140 and A/C will stop working. If engine is shut off cooling fans will continue to run. Is coolant temp info fed to instrument cluster and then ECM (bad gauge) or is temp info sent to ECM and then to gauge(bad ECM)? I replaced coolant sensor about  months ago and thought I had problem fixed but is back again. Any help would be greatly appreciated.

Hi Doug,

Serial data from the PCM tells the Instrument Panel what to do.  Since you have other strange temp related anomolies, I would not suspect a connection or IP issue.

Most likely, the PCM is getting incorrect inputs or has a defect.  To continue, you need special tools and expertise, to avoid making a costly mistake.  At least a scanner and good information system.

Let me give you a couple of links.  

This one will help you find a qualified professoinal to take care of this.  This would be the most cost efective option.  Shops on this engine are members of the International Automotive Technicians Network:

http://www.iatn.net/dir/search.asp

If you still want to tackle this yourself, you need information.  You can get access to systems information on line.  While this may not help you to diagnose the issue, it will give you an appreciation for the complexities of your ride and serve to illustrate the value of a pro in this situation.  Here is a link to ALLDATA DIY Online:

http://www.alldata.com/vehicle_owner

Thanks for the info. After further testing found new thermostat had failed in the open position which caused ECM to go into failsafe mode. Seams odd to quit sending temp data to gauge but new thermostat fixed the problem.

Thanks for the follow up, Doug.  The T-stat may have been sticking intermittently too.  Glad you got it taken care of.   ;)