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Is it under manifold I have had mine apart twice once because wrong part and the 2nd time I replaced sensor no luck with thermostat can't find it. You have to take off the engine( manifold off it's not the easiest job but can be done with the right tools
Can you update us with the outcome here? Did you find and install a thermostat? If GM, what was the part # ?
 

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Can you update us with the outcome here? Did you find and install a thermostat? If GM, what was the part # ?
I found this and shows back in stock again. Not sure if it is really a GM OEM part, but says it is made to GM specs.
Also says as of this writing that there are 7 in stock.
IT seems the part goes buy two different part numbers . . . both are on the GM label.

15-11122 and 12678779

GM Thermostat

Pipe Art Metal Tool Wood
 

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I think one number is from the ACDelco catalogue and the other is GM's part catalogue number. That's seen fairly often.
 

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Apparently it is GM 12678779 . . . . but also has 15-11122 on the OEM label .
You can try GMparts Outlet as suggested
. Perhaps they have it. . . or perhaps not.

Below is a video of the 2017 GMC Denali (not Limited models) with the LGX 3.6L engine.
This is the same as +2017 C1xx models to date so far as of this writing.

That thermostat is a task to get at so for future readers, this may help.

Wow what a stupid location for a thermostat. Might as well change the coils and plugs under the manifold as well. This job is going to suck. Having the same **** issue. It happened once and I just unplugged the battery and reset all the computers and it was gone for awhile. It came back today. This time I cleared all the codes and it went away and started working fine again. After seeing this it's time to just get in and knock it out
 

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Wow what a stupid location for a thermostat. Might as well change the coils and plugs under the manifold as well. This job is going to suck. Having the same **** issue. It happened once and I just unplugged the battery and reset all the computers and it was gone for awhile. It came back today. This time I cleared all the codes and it went away and started working fine again. After seeing this it's time to just get in and knock it out
How many miles were on it when the Thermostat issue arose?
And , yes a major pain to replace all of that. Don;t forget to replace the Engine Coolant Sensor right next to the thermostat also. Not worth have to to do that later at some point when it's a low cost part.

Problem . . .No A/C, Power Steering Assist Reduced/Disabled notification, Fan running all the time, Engine Temp at 0, and P0128 Code.
GM Original Part Numbers.
Manifold Gaskets x6 Part# 12648665
O-rings for the 2 return lines x2 Part#12648547
Thermostat assembly and water pipe Part# 12678779 ( Or GM Genuine Parts 15-11122 )
Engine Temperature Sensor (might as well replace this $12 part at the same time) *This one you can get anywhere Part# TX281
 

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Yeah I orded everything. Was about $350 for everything. Jumper harness, intake manifold gasket kit, 2 black o-rings, temp sensor, thermostat, 3 coils, 6 spark plugs. Still need couple coolant bottles.
Now anyone have the torque specs and order for everything? Also do the intake bolts need to be replaced if they are torque to yield bolts?
 

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Yeah I orded everything. Was about $350 for everything. Jumper harness, intake manifold gasket kit, 2 black o-rings, temp sensor, thermostat, 3 coils, 6 spark plugs. Still need couple coolant bottles.
Now anyone have the torque specs and order for everything? Also do the intake bolts need to be replaced if they are torque to yield bolts?
From the couple of YT videos I've seen they don't say the intake manifold bolts need to be new. The silicone rubber seals do the job so snug and even tightening should do the job just to a good snug down.
The Thermostat bolt torque is probably most important I would think.
 

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From the couple of YT videos I've seen they don't say the intake manifold bolts need to be new. The silicone rubber seals do the job so snug and even tightening should do the job just to a good snug down.
The Thermostat bolt torque is probably most important I would think.
From the couple of YT videos I've seen they don't say the intake manifold bolts need to be new. The silicone rubber seals do the job so snug and even tightening should do the job just to a good snug down.
The Thermostat bolt torque is probably most important I would think.
Haven't found the T-stat hiding yet
Intake manifold is 18ftlbs
 

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From the couple of YT videos I've seen they don't say the intake manifold bolts need to be new. The silicone rubber seals do the job so snug and even tightening should do the job just to a good snug down.
The Thermostat bolt torque is probably most important I would think.
Ill hook all of you up with the correct info!!!
 

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How many miles were on it when the Thermostat issue arose?
And , yes a major pain to replace all of that. Don;t forget to replace the Engine Coolant Sensor right next to the thermostat also. Not worth have to to do that later at some point when it's a low cost part.

Problem . . .No A/C, Power Steering Assist Reduced/Disabled notification, Fan running all the time, Engine Temp at 0, and P0128 Code.
GM Original Part Numbers.
Manifold Gaskets x6 Part# 12648665
O-rings for the 2 return lines x2 Part#12648547
Thermostat assembly and water pipe Part# 12678779 ( Or GM Genuine Parts 15-11122 )
Engine Temperature Sensor (might as well replace this $12 part at the same time) *This one you can get anywhere Part# TX281
Sorry at 56k miles right now. Its happened twice in the past few months. I usually just clear it and it goes away for a period of time. First time I pulled the battery cables to let everything reset amd it went away for about 2 months. Then it just happened again yesterday. This time I cleared it with my scanner and today so far it's good.
 

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So here is my problem with replacing parts with this issue. My scanner reads proper water temp but my cluster does not. The Thermostat opens and closes as it should. Luckily I had my scanner so I could activate my ac compressor and have AC on the drive down. If I can read all these temps and everything working on my scanner what is the issue with the computer not comprehending it's working correctly?
 

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So here is my problem with replacing parts with this issue. My scanner reads proper water temp but my cluster does not. The Thermostat opens and closes as it should. Luckily I had my scanner so I could activate my ac compressor and have AC on the drive down. If I can read all these temps and everything working on my scanner what is the issue with the computer not comprehending it's working correctly?
Are there two coolant sensors? Maybe one on or near the radiator?

If the cluster does not read correctly I would guess then the Engine Coolant Sensor right next to the thermostat may be the culprit or the connector/wiring to it.
I read where one owner checked went to check the connector to the sensor and it was loose and had some corrosion on the contacts.
It too , however is difficult to get at let alone just see it.
 

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I know my scanner was reading 165-183 was the highest it got cause I couldn't turn the fan off. Oil temp was good at about 19x* trans temp 165. Ill check the connector if I can and try a see something. But my scanner is reading ECT so IDK if there is an issue with the can bus or HMI.
 

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Just took the negative cable off for about a minute and put it back on now it's reading again. I Can't get my fat @$$ hand in that small hole. Wire seems fine but I ordered a new jumper anyway as well. Im just going to replace everything, since I bought everything, but I do not believe it's a part issue except for maybe the jumper harness.
 
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