ENGINE HOT - A/C OFF message on DIC

[PTpro]

It is also possible that you have the wrong thermostat fitted and it opens too soon. Some owners like to run their engines with a low opening temperature thermostat, I don't know if you know the full service history of your XLR. A technician should measure the temperature of the coolant when diagnosing that error message. Measuring the rate of warm-up can also be a strong pointer to a thermostat problem.

I've bought the car from a Cadillac dealer as a Certified Pre-Owned with only 1300 miles on it. It was never tampered with before I owned it, but I'm surprised that you say a low operating thermostat would cause an "engine hot" display????

 

[Rule62]

I've bought the car from a Cadillac dealer as a Certified Pre-Owned with only 1300 miles on it. It was never tampered with before I owned it, but I'm surprised that you say a low operating thermostat would cause an "engine hot" display????

You should scroll up and reread xlrlist01's post which explains this in clear terms.


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[onalaska]

The message "Engine Hot - A/C Off" on the DIC is really misleading when in this case it actually means the engine is not warming up to the correct temperature in a prescribed length of time. I suspect displaying the incorrect message on the DIC is a simple software programming error by GM. It may also display that message when the engine is actually overheating, I don't know as I have never had that problem.

If you really want to be totally confused just Google Engine Hot - A/C Off and see what you get. This same message has been appearing across a number of GM vehicles and creates a huge amount of confusion, apparently even for the mechanics at the dealers because I have read on some of the forums where they just throw expensive parts at the problem, even replacing the engine computer, while never trying the thermostat and never fixing the problem. After reading through many different forums with people having the same message I found a number of people that solved the problem by replacing the thermostat and that was the easiest, least expensive, and simplest thing to do first and it worked for me. I don't doubt that there may be other causes for this message to appear in some instances, but my guess is that the thermostat is the problem most of the time.

It looks like GM bought a bunch of defective thermostats all of which are used on many of their different models and they stick open occasionally and when they do the computer reports the the Engine is hot when it is actually the other way around and is not warming up like it should. I would expect that message if the thermostat stuck closed as the engine would overheat, not the other way around.

 

[xlrlist01]

[snip]
I suspect displaying the incorrect message on the DIC is a simple software programming error by GM
[snip]

It looks like GM bought a bunch of defective thermostats all of which are used on many of their different models and they stick open occasionally and when they do the computer reports the the Engine is hot when it is actually the other way around and is not warming up like it should. I would expect that message if the thermostat stuck closed as the engine would overheat, not the other way around.

I can't comment on if GM have a bad batch of thermostats, but the design which makes it most likely that when a thermostat fails, it fails open is clever. It is possible for a modern thermostat to fail closed, but that is rare.

If the DIC message is present, then P0116 should set. P0116 sets if "the actual change in the measured ECT is less than the change in the modelled ECT". (ECT = Engine Coolant Temperature). Putting this another way, it says, "if the engine doesn't warm up fast enough, set code P0116". The diagnostics for P0116 in order are:

1/ Perform the general diagnostic check (this is the standard first check)
2/ Check coolant level (that is pretty easy)
3/ Check thermostat. GM have worked this out and do direct to the quickest and cheapest fixes first.
4/ Then it gets to checking the sensor, the wiring etc.
5/ It is step 13 before the ECM gets changed.

GM are not displaying the incorrect message, the ECM cannot know if the thermostat or the sensor has failed, hence they display the more serious message. Perhaps it could say "Engine hot or thermostat stuck" and forget the "A/C off" bit. It would also help if the workshop manual linked the warning message to the diagnostics.

 

[Rule62]

I can't comment on if GM have a bad batch of thermostats, but the design which makes it most likely that when a thermostat fails, it fails open is clever. It is possible for a modern thermostat to fail closed, but that is rare.

If the DIC message is present, then P0116 should set. P0116 sets if "the actual change in the measured ECT is less than the change in the modelled ECT". (ECT = Engine Coolant Temperature). Putting this another way, it says, "if the engine doesn't warm up fast enough, set code P0116". The diagnostics for P0116 in order are:

1/ Perform the general diagnostic check (this is the standard first check)
2/ Check coolant level (that is pretty easy)
3/ Check thermostat. GM have worked this out and do direct to the quickest and cheapest fixes first.
4/ Then it gets to checking the sensor, the wiring etc.
5/ It is step 13 before the ECM gets changed.

GM are not displaying the incorrect message, the ECM cannot know if the thermostat or the sensor has failed, hence they display the more serious message. Perhaps it could say "Engine hot or thermostat stuck" and forget the "A/C off" bit. It would also help if the workshop manual linked the warning message to the diagnostics.

That seems to be quite logical.

I would add that the "A/C OFF" portion is really shorthand for "compressor off" as all three of the incidents I had seemed to occur when I was looking for heat. Also it corresponds to the "A/C OFF" button on the HVAC module.


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