'08 Alpha. Headed for work yesterday morning, and suddenly got the "AC OFF" message on the DIC and the temperature gauge went dead. Fired up my Torque app, and temps were fine, albeit a bit low (it was in the 40s outside, and I was only a few miles down the road). Watched my Torque app and the engine temps came up to normal (+/- 190), but the temp gauge never started working, nor did the AC. No set codes, but I did have a "Pending" code of P0128.

Got to work and did a few internet searches here and elsewhere; seems to be the general consensus that the temp sender or the thermostat is the likely culprit. Since the actual engine temps came up fine, I'm leaning toward the sender. Cleared the "Pending" code (it never went past "pending", and I never got a CEL), and things went back to normal, so far.

But here's what has me confused. Why was the temp gauge dead when the ECM/BCM was back to clearly reporting normal temps to the Torque app? I don't believe that there are 2 temp senders. My thought is that the sender had an intermittent malfunction, triggering the ECM/BCM to shut down the A/C and the gauge, but then started to work fine again. However, the initial triggered event caused the gauge and A/C to remain disabled, even after the sender was back to normal, until the pending code P0128 was actually cleared. Sound logical?

 

Try removing the BCM B fuse and replacing it.

 

At this point, everything is back to normal, so I'm going to just hang tight. It just seemed odd that while the gauge was dead, the ECM/BCM was in reality reading/outputting the correct temperature data.

 

The default PCM setting is to assume overheat when the temp sender signal is interrupted. So your though that the temp sensor may have malfunctioned for a very short time may be a good one, it would have triggered the beginning of overheat protection to show AC Off, but went no further because the sender started working again.

Check the connection & wiring at the temp sensor and keep an eye on it.

It could still be the T stat staying closed too long causing temp to come up too fast, but eventually opening and stabilizing.

 

Thanks, Doc. I'll check on the wiring at the sensor. I don't think the thermostat stayed closed, as the actual coolant temp was still low (about 140F) when I noticed the problem (just a few miles into my commute). The odd thing is that the gauge continued to be "dead" even after the sensor started working again, until the pending code was cleared. Interesting process for GM to use.

About 10-15 years ago, I had a 1989 Toyota 4WD van. Due to the lack of airflow through the engine compartment and the resulting underhood heat buildup, it was very hard on alternators, needing a replacement about every 12-18 months. Toyota designed the system so that if there was a problem with the charge system, EVERY light on the dash would come on, not just the "charge" light. I guess they really wanted to get your attention!