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Jared D
Jared D, ASE Certified Technician
Category: GM
Satisfied Customers: 1334
Experience:  Shop Supervisor at County Fleet
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I'm looking who knows the answer to the mystery thermostat

Customer Question

I'm looking for someone who knows the answer to the mystery thermostat malfunction in my 09 Chevy Silverado (and several other years and models of GM vehicles that also have the same problem per plenty of internet posters) that causes the thermostat to completely stop working randomly. I see that all kinds of people have errantly assumed they either needed a new thermostat (I tried that) or a new sensor (too many people have tried that before me I'll assume there is no need). From what I can gather a few people have also determined that if they completely disconnect their battery the gage will reset itself and work again. I notice mine just works when it feels like and doesn't when that suits it better. The downside is that the computer assumes the vehicle is hot and shuts off my A/C which is a problem when the ambient temperature is over 100. I know there are folks out there that have suffered the ungodly misery and indignity of paying to have someone get to the heart of the problem. This has occurred frequently enough on enough vehicles that I'm sure there is an answer if I can find the right GM expert that's encountered it. I can't afford the $2K experimental research project it will cost me at my local dealership where no one is familiar with the situation, plus the unknown (but frightening) potential repair costs. Given that stopping the electric flow completely resets the gage my assumption is this is either a wiring assembly or product problem or (OMG Please NOT THIS) a computer problem. You know, as in when all else fails try the restart button....If anyone has run across this situation I would be greatly appreciative to have some guidelines for my mechanic to get him moving in the likely direction of the problem.
Submitted: 5 months ago.
Category: GM
Expert:  Jared D replied 5 months ago.

I have seen this a couple of times but it always comes to one of two things, a bad thermostat or a bad coolant temp sensor. Here is how I would diagnose it. You will need a scan tool:

1. Check for a possible DTC P0128 set in the Powertrain Control Module (PCM).

2. With a cold engine, check the engine temperature and air temperature values in the PCM data. The values should match within a few degrees.

3. If the engine temperature sensor value does not appear to be accurate, repair as necessary.

4. If the engine temperature is accurate, replace the thermostat.

Expert:  Jared D replied 5 months ago.

Something else you might check:

Has the truck ever had the radiator replaced? I have heard of a couple of instances where the wrong radiator is used and the replacement radiator does not have a surge tank restriction orifice. The essentially makes a huge air pocket in the system which causes the false overheat.

Expert:  Jared D replied 5 months ago.

You can check this by looking to see if the coolant is quickly running through the overflow tank when the problem is occurring.

Expert:  Jared D replied 5 months ago.

Please let me know if you need additional info.

Expert:  Jared D replied 5 months ago.

Let me know if you need any additional help with this.

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