JohnTG

Hi,

I have a 2008 6 cylinder Chevy Silverado. The Engine Hot-AC turned off light has come on twice and the temperature gauge goes to where it was when I started the truck. This has happened twice while I was driving to work. The first time when I came back out to drive home it came on immediately when I started the truck up but the temp gauge went to about 180 after a couple minutes when it was till showing nothing. The gauge just popped to 180 instead of a gradual warmup and the light went out. Is this a computer problem glitch or is their something more serious going on. The engine could not possibly be hot when the truck just sat for over 8 hours in cold weather. Any help would be appreciated.

John

masabe

John,

I have the exact same problem with my 2008 GMC Sierra. Did you solve the problem? IF so what was the fix?

Chevrolet Customer Service

I apologize for any inconvenience with this issue. Have you contacted the dealership? I would recommend you to talk to the dealer and see if they can troubleshoot the problem. Also, is the vehicle still under warranty? Let me know how it goes.
Breanne
Chevrolet Customer Service

masabe

The vehicle has 62000 miles and the dealer wants $100.00 just to check it out. This problem is all over the internet, I am sure they must know what causes this and can tell me what the problem is. I have now taken a reading of the resistance of the temperature sensor when the problem is not there. I will compare that to the resistance when the problem happens. I am hoping that will tell me if it is the sensor or not.

wgf

you can add me to the list of folks with this problem.

2008 silverado , 4.3L engine with approx 50,000 miles.

Things that i can confirm.
-the engine is not running hot ( 205-210 F)
-the sensor works fine ( the old one and the new one i replaced it with)
have the same resistance at operating temp
- the connector and wiring to the ECM do not appear frayed or corroded and always show good ohm values.
- the coolant levels are full and well purged.
- when the error message appears the temp gage is deactivated even though the sensor is still functioning and the radiator fans are powered (clearing the codes immediately returns the temp gage to prior level)
-The ECT sensor provides input for both the ECM and temp gage on the dash.
(i believe the ECM (control module) is deactivating the dash temp gage )
- if i disconnect the ECT sensor, it will generate the same error message.
after reconnecting the ECT (temp sensor), the message clears, temp gage, radiator fans and A/C functionality are restored.
not clearing the code after the error message occurs eventually creates an OBD error (P0128) along with 'check engine light'. At this point, Freeze Frame data read by scanner says ECT temp is too low (167F) . I'm sure that is because both radiator fans are maxed out as soon as the ENGINE HOT message first appears and the thermostat can't keep up.

Things i'm not too sure about:
-Changing the thermostat seems to work.
despite the fact that it raised the ECT by a approx 5 degrees to 210F.
at present I've only made a couple of round trips to work since replacing the thermostat w/o any nasty messages. However, prior to the change, i was tripping the ENGINE HOT message on both legs of my commute for over a week.
_so i am hoping that was enough to make the problem go away.
the message appears to be a bogus programed response to the warm up cycle of the truck. either the thermostat has been steadily failing or the recent colder outside temps have kept the ECT from reaching some predetermined level in the ECM. or worst case the ECM is the part thats the ultimate cause of all this.
I'm keeping my fingers crossed for the thermostat.
-bill

EinST

Welcome to the forum. Thanks for an excellent and intelligent account of your investigations. One thing I'd like to add is, the engine coolant temperature gauge in the I/P cluster in your vehicle no longer reflects the instantaneous temperature (it hasn't for several years). When it used to do, a lot of folks found the wild swings in the needle distressing. Thus, the gauge was made to register artificial, modeled temperatures.

Another thing, the ECTS sensor and wiring to the PCM constitute only half of the ECTS circuit. In order for the voltage drop across the ECTS to be measured accurately, the PCM connector should not add appreciable resistance on the circuit and there shouldn't be anything funny going on on the voltage divider circuit inside the PCM. I wonder if things weren't slightly out of calibration, including the thermostat, for the temperatures to be derived lower than the threshold in your case. If so, the new thermostat may indeed be your answer.

When you said "not clearing the code after the error message occurs eventually creates an OBD error (P0128)," what code were you clearing and was P0128 pending at the time?

wgf

Hi EinST,
thank you for the response.
I don't truely understand the program logic for the ECM (PCM?) when it decides to activate the "ENGINE HOT.." message. But i do believe that the temps displayed on the dash temperature gage is/was correct before it was deactivated by the ECM.
I was not smart enough to just put a thermometer in the radiator, but the relatively low pressure in the coolant system and the fact that i was not scalded while handling/draining fluid for the sensor swap suggests that its just not that 'hot'.

regarding my statement about clearing the code:
you are correct that the "Engine Hot" message does indeed trigger a pending P0128 (ECT temp is LOW). That is what I have been clearing , with the scan tool , as soon as the message appears. However, at this point i dont think the ECT temp is too hot or too cold. If i do not clear the error message ( and OBD P0128) I believe the coolant temp does slowly drop to the value indicated in the freeze frame (167F) when the "CHECK ENGINE" light eventually comes on. Thats just speculation on my part because the radiator fans are maxed out as long as the "engine hot" message is active.

If the new thermostat does not solve my problem, i will hook up a scan tool that lets me log ECT temps as the various events unfold. I never really expected this to be a science project but it would be nice if the various messages, codes and control modules would get their 'story' straight.
regards,
-bill

EinST

Well, I think you should consider P0128 to be the reason for "ENGINE HOT A/C TURNED OFF" and not the other way around. P0128 is when the engine coolant temperature is deemed to rise too slowly. In the absence of ECT-specific trouble codes, an ECTS or cooling system fault is assumed and thus all bets are off. The PCM then does things to protect the engine.

The P0128 rationality check is run when the ECT at startup is less than some temperature (depending on the IAT). The ECT is then watched until it reaches some other temperature (again, depending on the IAT). The latter is below the thermostat opening temperature (no coolant flow through the radiator) and thus the ECT can be reliably predicted. This is why I mainly suspect the ECTS circuit but I guess I can also see the possibility of the thermostat malfunction in older vehicles.

Your idea of logging the ECT sounds excellent, especially what the PCM thinks the ECT is when "ENGINE HOT A/C TURNED OFF" is displayed. Again, thanks and I'd appreciate it if you'd keep us updated.

wgf

Thanks,
that does make more sense.
I admit i have been trying to make the details justify the "ENGINE HOT A/C TURNED OFF" message.
And... the results so far do suggest that i should stick to the P0128 fault.
Especially since the only defintive mod i have made to the cooling system is raise the temp, not lower it.
one more day (for a total of 2 with no faults.
cross your fingers,
-bill

masabe

Finally got some intelligent response on this problem. This is all making sense to me. I am going to change the thermostat this weekend. I will also reply with my results. Has the problem still not returned after changing the thermostat? thanks