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Discussion Starter · #1 ·
This morning as I parked at work my AC cut out and blew hot air. Not totally unusual, usually kicks back on after a bit. Come this afternoon to go home still
No AC, and my coolant temp gauge didn’t work. Then as I was almost home, my check engine light came on?? Other than that, the truck seemed to run fine, made it home ok. Turned the truck off and the engine fan ran for a few minutes.

Recent work: nothing major, new battery and topped off coolant level.

Any ideas??Oh my OBD scanner errors out, so I wasn’t able to pull any codes.
 

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This morning as I parked at work my AC cut out and blew hot air. Not totally unusual, usually kicks back on after a bit. Come this afternoon to go home still
No AC, and my coolant temp gauge didn’t work. Then as I was almost home, my check engine light came on?? Other than that, the truck seemed to run fine, made it home ok. Turned the truck off and the engine fan ran for a few minutes.

Recent work: nothing major, new battery and topped off coolant level.

Any ideas??Oh my OBD scanner errors out, so I wasn’t able to pull any codes.
Could be the temp sensor. When it goes the AC won't work also, it's the truck trying to save itself.
 

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Discussion Starter · #4 · (Edited)
OBD scanner gave a P0128 code, Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). I cleared the code and for some reason it worked after that. Will see if i get the same symptoms tomorrow and if the code comes back, will keep you posted.
 

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OBD scanner gave a P0128 code, Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). I cleared the code and for some reason it worked after that. Will see if i get the same symptoms tomorrow and if the code comes back, will keep you posted.
Ahhh, ok, that means the t-stat is bad, stuck or sticking open. It won't heat to operating temp. This will kill the AC too.
Watch your temp carefully, I think you will find it won't heat up.
 

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Ahhh, ok, that means the t-stat is bad, stuck or sticking open. It won't heat to operating temp. This will kill the AC too.
Watch your temp carefully, I think you will find it won't heat up.
why do you say that would kill the AC also, the AC condenser should work across the coolant temp operating range right? I'm still betting on air pocket, which would fool the sensor into thinking that it was below temp range. it's a sensor that is typically wetted and gets wonky readings in air space. even if the thermostat was wide open, the coolant temp would not be "below range" at any time. coolant is cold at startup right?

Additionally, thinking critically, if the OP needs to run the AC, we can assume the abmient temp is pretty high, so the tstat would normally be open reasonably wide, so a temp sensor reading below range doesnt make sense there either.

just thinking out loud.
 

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why do you say that would kill the AC also, the AC condenser should work across the coolant temp operating range right? I'm still betting on air pocket, which would fool the sensor into thinking that it was below temp range. it's a sensor that is typically wetted and gets wonky readings in air space. even if the thermostat was wide open, the coolant temp would not be "below range" at any time. coolant is cold at startup right?

Additionally, thinking critically, if the OP needs to run the AC, we can assume the abmient temp is pretty high, so the tstat would normally be open reasonably wide, so a temp sensor reading below range doesnt make sense there either.

just thinking out loud.
Simple, they program it this way. I don't know for sure why but my guess is so you will fix it. Could be also because they don't want the AC on if the cooling system is in question.
Normal operating temp is crucial for the system to work correctly and for emissions of course.
When I say it kills the AC I mean the PCM kills it. Nothing to do with the AC system other than the computer controls.
 

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Not necessarily IMO. I think he had an air bubble in the closed system since he "topped off the coolant".
could be but my guess is stuck open t stat. "topping off" to me means an inch or two in the bottle.
This code also manifests over time, a bubble or two causing a bad reading won't do it. Plus when a system gets that low it overheats, the air in there is hot too.
 
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even if the thermostat was wide open, the coolant temp would not be "below range" at any time. coolant is cold at startup right?
If the t stat is suck open the coolant won't get to normal range, so yes, it will be below range. The code happens when it sees the truck never warm up all the way.
 
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If the t stat is suck open the coolant won't get to normal range, so yes, it will be below range. The code happens when it sees the truck never warm up all the way.
so does the sensor wait somehow until the truck is supposed to be warmed up? because obviously when you start the truck in the morning, on a cold day, even if the coolant is at 20 degreesF, that's still "normal" temp at startup. unless there is some delay in the system that throws a code if the truck doesnt reach operating temp after x minutes of operation?
 

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so does the sensor wait somehow until the truck is supposed to be warmed up? because obviously when you start the truck in the morning, on a cold day, even if the coolant is at 20 degreesF, that's still "normal" temp at startup. unless there is some delay in the system that throws a code if the truck doesnt reach operating temp after x minutes of operation?
Yes, the PCM watches the temp and it knows it should warm up in a certain amount of time. If that does not happen in a predetermined amount of time over so many drive cycles it will set the code and shut off the AC. That would be why clearing the code got his AC back. I bet it will return in a week or so depending on how much he drives or course.
"normal" temp means full operating temp, most cars nowadays are right around 195 deg. (whatever the stock t stat is set for)
 

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I have seen some thermostats at 203 degrees on the newer vehicles.
 
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I have seen some thermostats at 203 degrees on the newer vehicles.
Yes, they have gotten hotter over the years, emissions .....
before computer controls 185 was the norm.
I still remember some older GMs like in the 90s would not even turn on the fan until 230 degs! Running the AC kept them cooler because the fan ran with it on. I always thought that was kind of crazy.
 

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I remember 160/180 in the 70’s. 203 is just a little to close to boiling for my comfort.
 

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Discussion Starter · #20 ·
Just wanted to follow up on my post. I have not encountered the symptoms since I cleared the code. I also seem to be operating at normal temps, I would guess the line is at the 195-200 degree mark.

And to clarify my topping off of the coolant comment, i probably put in about 8-10 oz, it was sitting about 1/2-3/4" below the line. Not sure if that helps determine the cause or not.
 
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