I have a 2009 base that I drive daily. Recently I tripped an "Engine Hot/No AC" error on the DIC and the temp gauge read dead cold. Scrolling through the info screens the temps for oil and trans were fine. So I replaced the coolant temp sensor, along with the pigtail, which was crumbled at the plug. Disconnected battery for 10 seconds, reconnected, and all was fine. Fast forward to this past weekend. Each morning on my way to work at @4:30am I get 5 miles into my commute and the "Engine Hot/No AC" pops up again. So I check the wires at the sensor, disconnect the battery again, etc. All is well again. Until the next morning when the same thing happens again. There are no codes when I scan the system. The only time it trips is early morning when it's cold since the temps have started dropping. It doesn't trip on the way home, or when I drive the car later in the morning on my days off.

Any ideas what would cause this?

 

Either air bubble in the system still that is causing the sensor to show over heat when the coolant itself is not that hot, problem with the system like the T stat not opening up and causing the temp to go over temp until is does start to open up, or you need to trace the wires all the way back to connectors to see if still have a wire problem, or connector wiring problem at end of line that is causing a short to cause the motor to think that is over heating isntead. Also, start motor in the morning, turn the A/C on and make sure that the radiator fan is running, even with the motor dead cold. If fan does not come on, then check raditor fan connector at the side of the radidtor to make sure it not burnt.

Also sent DIC to temp to watch the coolant heat up rate to start with ,and should point you in the direction that you need to check. Very fast major spike and drop during lower warm up, would guess shorting in the wiring for head coolant sensor. temp coming up normal, fan kicking in, but temp not stabilizing and continuing to climb past say 220, then maybe T stat sticking closed.

 

In the morning, as soon as I start the car, the coolant temp reads normal, temperature increases as normal during warm-up. Never gets over 195 on the trip to work, as it's all freeway and no traffic. Hottest the oil temp gets on the trip in is under 200 as well. So the engine is not actually hot, something is telling the computer the sensor is out of range. It may be that the new temp sensor is faulty. The fan does come on in the morning when I turn on the A/C.

I would consider a wiring issue if not for the fact that it doesn't happen on the way home from work, or when running errands throughout the day. Something unique is happening only in the early morning to cause this, and it's literally driving me insane...

I seem to remember reading somewhere about the computer comparing coolant temperature with ambient air temperature via the intake air temp sensor to trip an error code, but I have no codes stored or pending when I scan the ECM. It just seems to me that something is causing the computer to disregard the coolant temp sensor input entirely, based on the gauge reading dead cold (XXX on the DIC), yet functioning normally after disconnecting the battery.....

 

I also daily drive my 13 base vette and I’ve had the same issue 3 times now. First time fan pigtails were melted and fixed that, second time fan works fine and seems to be working overtime the way it was constantly on and stuff, temp wasn’t being read.. also said Coolant temp is XXX AGAIN fixed that issue replaced the temp sensor now for the third time I have the same issue. Driving home from work and car randomly says engine hot a/c not working I hear the fan again going crazy and I rush home to turn it off (I’m 5 mins away from home). So I’m here with you if you find a fix! I’m going to go buy a new sensor right now to replace the old one again (the old one has only a few months in but idk what else to do gonna scan it once the new sensor is one hopefully it fixes the DIC screen so I can feel comfortable driving it to get scanned !

 

wonder if your fan connection is melted

 

I think if the fan connection is melted no more power would be going to the fan? I could be wrong though but just something I noticed when it first went out. Right now fan turns on immediately and stays on

 

A/C is driving that for the most part when on (needs to keep the fan running to dray air through the condensor), or could be cold and the ecm is turning it on the A/C as well, to dehumidify the air so you don't get inner frost when the heater first starts up.

So if warmer, then turn the A/C off via the HVAC controller with the engine not warmed up yet,and see if the radiator fan is still coming on when you start the motor.

As for back to original problem, car monitors the temp as it comes up,and if a wild fluctuation, like air in the system that gets to the temp sensor in the head, that is what trips the problem with wild fluctuation as it coming up/not holding steady enough in its rise and fall temp rates.
So could be sensor or wiring problem, or could be air in the line, or even leaking head gasket that is pushing air in to the system, if problem is not that that actual temps of the motor are going sky high and staying there instead. As for sky high and staying there, then something blocking the system to not allow the fluid to flow like sticking closed t stat, water pump impeller no longer fixed to the shaft, RTV glob blocking the passage, or could be fan not coming on instead.

 

Have you check for Coolant? In the engine not the bucket.

 

The problem isn't the coolant temp sensor. It's the engine temp sensor. It's located right under the exhaust manifold on the front left side of the engine.

Tom

 

at Auto part stores they may have the same name,(coolant sensor) just ask for the one that "talks" to the computer NOT the Temp gauge. Remember a lot of parts counter people know computers but have NEVER worked on a car in their lives. Just try asking for valve cover gaskets for a -small block Chevy- LS engines are NOT small blocks, they have near the same cu.in. but are entirely different. The computer does not know this, that is why they need to know : year, make, and model. Parts people become better at what they do when those of us that are "Car Guys" / "Car Girls" help them along and teach them a bit. When you help them you and everyone they deal with gets better service!