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Discussion Starter #1
Temp gauge on my 07 3.5 vert stopped working this morning and AC isn't kicking on. From reading seems like this can be temp sensor or stuck open thermostat. There's no temp reading on the gauge at all so I'm betting on a bad sensor, but thermostat is cheap so I'm gonna replace it too while I'm in there. Thinking it will be easiest to pull the passenger headlight so I can get my arm through that way to help. Have you guys learned any other tricks for replacing the thermostat on this? Every replacement I've done has always been on the top and front of the motor (SBC's and an LT1) so I'm curious to see if there's anything different I need to do to avoid leaks from this thermostat after replacement.

Thanks!
~Ken
 

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hey just a quick idea. I’ve got an 07 Malibu that has had this on going problem for years. After replacing both thermostat and sensor a few times I came to the conclusion it’s probably the ECM. Pull your battery cable for ten seconds and see if your temp gauge works. If so it can’t really be a stuck open thermostat. But still possibly a sensor. And the sensors on the back of the engine. I’ve never had to pull the headlight but might make things easier. Getting all the air out of the system after refill is always a pain in the ass for me.
 

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Get an OBD scan. After that reset the code. See if the temperature gauge works properly. If it does then it's probably the thermostat.
 

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I guess that’s the same as disconnecting the battery. But out of curiosity, why would that lead to the thermostat being bad? I had thought that the thermostat isn’t really controlled by anything but the temp of the coolant. So clearing a code should not have any affect on the actual thermostat.
Like I said I have had this on going problem for years with a Malibu. So I’m always looking for a solution.
 

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Discussion Starter #5
I pulled the negative cable for a few seconds and reattached it. Voila, presto no-problemo. It is weird to me too that the thermostat has been cited as causing an issue like this. Thermostats are responsive to the temp of the liquid they're in which is why you can pull them out and test them in a pot of boiling water - so not sure how they would affect the coolant sensor, especially with the two being on completely opposite sides of the vehicle. The car didn't seem to warm up slowly after the temp sensor started working, so I don't think the stat is stuck open. I also ohmed out the sensor and didn't get infinite resistance so I know it's not a broken coil in the sensor. Checked ground for the connector too and it has a good ground, moving the wires around didn't affect it. I did not get a CEL at any point.

The fact that I was getting absolutely no reading from the sensor earlier (not a low reading, just no reading at all) really makes me think it would have to be an electrical issue. I can't really think of how an issue with coolant temp would keep the sensor from sending a reading at all. The fact that disconnecting the battery and reconnecting it fixed the symptoms (likely temporarily) kind of reinforces that for me and makes me think ECM. And yet, for some reason, replacing the thermostat is said to be a fix for this.

The next time it happens I'm going to jiggle the harness around for the temp sensor first to see if that changes anything and move up the harness until I can't follow it anymore.
 

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The thermostat is completely mechanical while the temperature sensor is electrical. That's why scanning and resetting the code will let you determine if the sensor is at fault. If everything works fine after resetting the ECM, then it is most likely the thermostat. If not, then it could be the sensor. Even though the thermostat is mechanical, the ECM can measure its effectiveness and if there is a problem, it will disable the sensor, which makes it look like the sensor is defective even though it may not be.
 

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Huh, that's kind of a whack way for them to design that. It seems like it would be a better idea to flash a code than to disable the temp sensor. Is there a benefit to disabling the sensor, or is it just one of the things they did that we scratch our heads over afterward?
 

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They designed the system to default to the safest mode possible. When the ECM detects a coolant temperature error, it doesn't know whether its a faulty thermostat or a faulty sensor or something else. So to prevent giving a false temperature gauge reading, it disables everything completely and leave it for you to figure out the true cause. It is a whacky way to do things because it tricks you into thinking that the engine is cold when it could be overheated.
 

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They designed the system to default to the safest mode possible. When the ECM detects a coolant temperature error, it doesn't know whether its a faulty thermostat or a faulty sensor or something else. So to prevent giving a false temperature gauge reading, it disables everything completely and leave it for you to figure out the true cause. It is a whacky way to do things because it tricks you into thinking that the engine is cold when it could be overheated.
My car did not behave this way when I had a fault. Where did you get this information? Can you provide a link to your source?
 

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I don't have the source anymore. But any GM diagnostic procedures should describe the default factory programming. I believe I even came across it either here or other Pontiac forums. Your code may be different. Look for a code that indicates "Coolant temperature warm up period outside of parameter (P-0128 I think)", where the ECM will disable the temperature sensor, the temperature gauge, the AC system, set a fault code, and activate the coolant fans.
 

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But any GM diagnostic procedures should describe the default factory programming.... Your code may be different...
Exactly. So unless we have reliable information to show the programming for the OPs vehicle, we can't say for sure. As I said, my 2008 does NOT behave as you described.
 

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GM has a Technical Service Bulletin in 2007 for the Pontiac G6 with regards to Coolant temperature code P0128. An OBD scan should reveal whether your vehicle has this particular problem or not.
 
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