Engine Hot Warning/AC Lockout

Well here's a new one. Got a notice in the DIC that the engine is hot and AC is turned off. The temp gauge goes to zero. Now I'm driving in 60 degree temps and not using AC. There's no indication the engine is actually hot - anti-freeze is full, no discernible leaks. Parked the truck at work. Three hours later, message is the same and I drive across town to an eye exam. Two hours later, I leave there to go back across town to my mechanic's shop; a couple of miles into that trip, the code resets, the AC works, and all is well. DIC normal. Engine temp normal. The thrown code indicates a bad thermostat so we replace the thermostat yesterday. Driving back through the countryside from my daughter's house at a leisurely 50-ish mph in mid-50s temperatures this eveing and the code goes off again. Temp gauge to zero, message in DIC. Message can be overridden but comes back when you restart the truck and will probably be on tomorrow when I head to campus.

We'll work on it Thursday but does anyone have an idea what may be going on? Is the safety lockout on the AC an issue? There's nothing to say the new thermostat isn't DOA, but I'd doubt it. Thoughts?

Thanks in advance.

Odd that the gauge reads cold yet your DIC message says engine hot. Perhaps the signal reads hot briefly before going cold?

My guess is something wrong with your engine coolant temp sensor or wiring..

You're close, Mike. Found a bit of info over on the silverado/sierra forum. Seems mild (neither hot or cold) temps screw with the two coolant temp sensors (one for the gauge and one for the ECM), causing the ECM to assume a failure when it sees a delta T across the two sensors. AC/Delco or GM original thermostat helps with most of the issue, as the "good enough" generic 'stats apparently run outside of the ECM's desired range. Not sure whether to get AC/Delco or GM as AC/Delco 'stat runs at 190 degreeds and the GM 'stat runs at 187 degrees. If it's that picky, it needs to be correct.

Perhaps just get a scan gauge of some kind that can read the coolant temps on the fly and reset the code? Suggestions on brand/features? Since it seems the coolant system operates as it should, my biggest issue would be running without AC for cooling or drying air when running the defroster.

Interesting! Reminds me of the block heater, where too much temp difference after shutdown/startup will set a code...
(But not the factory block heater, which shuts off at low temp)

Seems to me, if it was just a delta T thing, that the gauge shouldn't zero out?
Also, if the thermostat was the issue, you would think everyone with a low-temp thermostat would have issues?
(maybe they do?)

Looking at 2008 wiring diags, the diesels have two temps sensors, but I only see one for gas V8.
(in the Engine Controls section) I can't find a separate gauge coolant temp sensor.
Only the ambient temp sensor, which may affect how the ECM performs it's checks... but you can see if outside temp is reading accurately..



If you have a scanner that can take logs, you could see a history of the temperature. But as frequently and continuously that it happens, you could catch the coolant temp when it occurs. The guage going to zero could be a separate gauge issue, so it wouldn't hurt to get actual ECT readings.

Guessing you had the P0128 code, which says engine not upto operating temp.
If your temp sensor is reading cold after warmup (like your gauge is saying), then I can see that happening...

PO128 is what we're getting. Gonna try two new sensors tomorrow. The thermostat we put in yesterday is for 187 degrees, which would match the replacement GM so I'm going to leave it alone for now.

Of course, it threw the same code again last night and kept displaying it this morning on startup but when I stopped by the mechanics to drop off some parts and restarted the engine, everything had cleared and stayed normal all day.

New temperature sensor went in today. We shall see...

I thought there was only one coolant sensor with 2 outputs?

Doubeleive said:

I thought there was only one coolant sensor with 2 outputs?

Click to expand...

I've heard it both ways but in my case, we could only find one sensor and yes, there were two outputs.

Haven't looked at mine yet.. Do you mean two wires? Or are you saying 4 wires?
Here's the diesel diagram, and it *looks* like two sensors..