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Discussion Starter #1
I noticed in the last day or two that my temp gauge was lower than normal. I ran an errand after work today and it started rough when I left. A few miles down the road it finally threw a code, P0128. I checked the coolant when I got home and the level was fine, but it was cold.

Seems like a sure-fire thermo failure. Anything else I could suspect?
 

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Could be the thermostat, faulty temperature sensor or the wiring circuitry. Checking the wiring, replacing the thermostat or temperature sensor might solve the problem. Does the heat gets hot versus a warm/cool heat if that makes any sense.
 

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Discussion Starter #3
I have been adjusting the temp more then normal.
 

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Inadequate coolant flow could be contributing to the issue, but shouldn't trigger the P0128 code. If you do decide to change the thermostat, wouldn't hurt to run a little water through the engine using a water hose. Of course, the lower radiator hose would have to be disconnected and the water pressure should be around medium to medium-low. Various manufactures produce coolant system flushes, but have yet to use any of them.
 

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Discussion Starter #5
I've had to replace the coolant temp sensor before, on the driver side head. Is there a second one? If flow were restricted, I'm thinking the engine would be more hot than cold.

This morning, the red X contact dealer warning went away after a few minutes. I didn't clear the code after I scanned it yesterday. Figured it would keep popping up. It did smell rich the whole drive.
 

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Went back and reread the original post i.e. temperature gauge was lower than normal so disregard the coolant flow portion. To the best of my knowledge, the G8 has only one coolant temperature sensor/sender. Perchance which brand was used as replacement?
 

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Discussion Starter #7
ACDelco
 

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Discussion Starter #8

When the sensor failed before, the gauge went to zero. It seems functional this time.
 

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Okie dokie. The temperature sensor has no correlation to the coolant being cold nor the P0128 code. Therefore, it seems replacing the thermostat would be the next step.
 
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Discussion Starter #10
Tis already ordered.
 

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Well, at least the thermostat on the GT is in a sensible location and easy to replace. The thermostat on the V6 is on the back of the engine and requires quite a bit more effort to change. Interesting "logic" since the car has a water outlet on the front attached to the upper radiator hose. However, most of that is neither here nor there and hopefully the new thermostat will remedy the issue.
 

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Discussion Starter #12
I replaced the thermo a few days ago. The code didn't return and the temp seems to be normal now. But it still starts slow/hard when warm. It is a long crank, then it slowly fires up to normal idle speed. What would cause that? It only started at the same time the thermo symptoms showed up. What would cause that?
 

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Was the check engine light cleared after replacing the thermostat? Would think a slow crank signifies a deteriorating battery, starter, or poor electrical connection. However, stranger things have happened.
 

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Discussion Starter #14
It was cleared. I've scanned again to make sure it wasn't pending, and it was all clear.
 

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For starters (no pun intended), would check the battery ground bolt Battery Ground Bolt (Read and check yours!!!!) and the starter's electrical connection(s). Ensure the battery post connections are solid and is this the original starter?
 

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Discussion Starter #16
The starter is the original. The battery was replaced a few years ago. It will probably be next weekend before I can get under and check the starter.
 

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Discussion Starter #17
I saw my temp gauge was flatlined today, and a few minutes later the check engine light popped on. Checked the codes and found p0128 again. I cleared the code and the temp gauge started working again. I thought that was odd. I replaced the temp sensor several years ago. Could the sensor be intermittent?
 

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Perchance was the replacement coolant temperature sensor an AcDelco professional or AcDelco GM original equipment? Am learning the difficult way regarding their professional line.
 

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Discussion Starter #19
It was the "original equipment" version.
 

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It says that an engine that has been off for 8 or more hours, the ECM expects to see IAT and ECT resistance values that will equal a temperature that is within 10 deg of each other. When the engine is started, it runs a test to verify that the coolant temp is rising as expected based on run/drive time. If the ECM detected a resistance spread (that is outside of the expected values) between the IAT an ECT, it will set that P0128 fault code. Since your engine coolant reading is so erratic, it sounds like the ECT sensor has lost its ability to produce the correct resistance value vs the actual engine coolant temp being sensed. If the engine temp stays pegged cold for an extended time period, that resistance signal going back to the ECM is being detected as a super cold temp. That might very well tell the ECM to provide a very rich fuel mix at a time when it shouldn't.
A quick test (in my mind) would be to disconnect the ECT connector at a time when its reading around its normal running temp location on the gauge. With it disconnected it should peg cold. Then jump the 2 pins an it should peg full hot. That would pretty much eliminate the ECM (which supplies the power an ground return).
 
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