Temperature gauge was working intermittently, code p0128

Welcome to 2CarPros.

Here is the definition of the code:

DTC Descriptor
DTC P0128
- Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

What that means is the engine isn't reaching operating temp at all or not fast enough. Chances are the thermostat is stuck open. In most cases, you will not notice anything different when driving, but the engine emissions are affected.

Anytime I have had this code, replacing the thermostat fixed the problem. That is what I suggest for this case.

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Here are the directions for replacing the thermostat. The attached pics correlate with the directions.

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2009 Pontiac G6 V6-3.5L
Removal and Replacement
Vehicle Engine, Cooling and Exhaust Cooling System Thermostat Service and Repair Removal and Replacement
REMOVAL AND REPLACEMENT
Engine Coolant Thermostat Replacement (LZ4/LZ9) (3.5L, 3.9L)

Removal Procedure


pic 1

1. Drain the cooling system. Refer to Cooling System Draining and Filling (LY7, LZE, LZ4, LZ9) (See: Cooling System > Removal and Replacement)Cooling System Draining and Filling (GE 47716 Fill) (See: Cooling System > Removal and Replacement)Cooling System Draining and Filling (LE5 Static Fill) (See: Cooling System > Removal and Replacement).
2. Remove the air cleaner outlet duct. Refer to Air Cleaner Outlet Duct Replacement (See: Air Cleaner Fresh Air Duct/Hose > Removal and Replacement > Air Cleaner Outlet Duct Replacement).
3. Reposition the radiator outlet hose clamp at the thermostat housing.
4. Remove the radiator outlet hose from the thermostat housing.


pic 2

5. Remove the thermostat housing bolt/stud.
6. Remove the thermostat housing and gasket.
7. Remove the thermostat.
8. Clean the gasket surfaces.

Installation Procedure


pic 3

1. Install a NEW thermostat.
2. Position a NEW gasket and the thermostat housing to the engine block.

Caution: Refer to Fastener Caution (See: Vehicle > Vehicle Damage Warnings > Fastener Caution).

3. Install the thermostat housing bolt/stud.

Tighten the bolt/stud to 10 Nm (89 lb in).


pic 4

4. Install the radiator outlet hose to the thermostat housing.
5. Position the radiator outlet hose clamp at the thermostat housing.
6. Install the air cleaner outlet duct. Refer to Air Cleaner Outlet Duct Replacement (See: Air Cleaner Fresh Air Duct/Hose > Removal and Replacement > Air Cleaner Outlet Duct Replacement).
7. Fill the cooling system. Refer to Cooling System Draining and Filling (LY7, LZE, LZ4, LZ9) (See: Cooling System > Removal and Replacement)Cooling System Draining and Filling (GE 47716 Fill) (See: Cooling System > Removal and Replacement)Cooling System Draining and Filling (LE5 Static Fill) (See: Cooling System > Removal and Replacement).
8. Inspect the system for leaks.

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Let me know if this takes care of the problem or if you have other questions.

Take care,
Joe

This am remote start will not work. Still same answer. Sorry to ask another?

Welcome back:

No problem. Roxanne, have you checked the battery in the fob? Here is a quick video showing how it is done:

https://youtu.be/Z6pUn73GvN4

Now, if the fob is working properly except for the remote start, I need to know if this has a factory installed remote start. If it is an aftermarket one, I won't be able to find schematics relevant because there are many different types, designs, and they all have a different way of installing.

Let me know if I can help.

Take care,
Joe

I bought the car from dealer with remote start. So i believe it is the factory one. The dealer says while check engine light is on the remote start will not work. This am light still on, my temperature gauge worked for a few miles then went flat. This afternoon flat for a few miles then worked. Would the thermostat, which is the codes i get make this happen? Thanks again.

Welcome back:

The thermostat is nothing more than a mechanical device that reacts to temperature. Although it is not working properly, I doubt it has anything to do with the electronics. I attached a portion from the wiring schematic. What I would suggest is to check the temp sensor for both power and ground when it isn't working. If you find fault with the ground, confirm the connection between the orange/black wire and tan is not corroded or damaged. Also, make sure the connectors at the PCM are not corroded or loose.

Roxanne, I don't know if you feel comfortable doing these things. It can get a bit involved. However, since it works on and off, I suspect it is a weak wiring connection. Here are a few links you may find helpful:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

Please let me know if I can help or if you have other questions.

Take care,
Joe

So today, my check engine light is off....omg what the heck

And my heat and air has worked fine even though light was on. I have appointment to have thermostat fixed. But if that is what is wrong, why would check engine light just go off??

Welcome back:

Good question. It could be that the t-stat was stuck and finally moved again. One thing about cars, they all seem to have minds' of their own anymore.

Let me know if I can help or if you have questions.

Take care,
Joe

All gauges go up and down. When I start my
car, but the temp gauge does.not go back up
my car is using alot of gas. I do have heat
any suggestions?

So the temp gauge just stays on dead cold? Also have you checked for codes yet? If not I would do that and let me know the exact code numbers not code descriptions?

You could have a bad coolant temp sensor or wiring/connector to it.

I have not got a diagnostic check. Don't know how to get a code

If you don't live in California you could go to a auto parts store for a free code scan let me know the exact code numbers not code descriptions? But if the check engine light isn't on they might not be able to get the codes with there do it yourself scan tools they use.But it's worth a try you have to use a code reader or more advanced scan tool.That you hook up to your diagnostic link connector on the bottom of your dash.

Here's the link to tell you about code reading.



https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

I recently had an issue where my check engine light came on and it was showing that it was the temperature sensor so I got it replaced. Everything was fine. Soon after I changed the radiator because I accidentally made a hole in it by running over something. Everything was fine again. About a week after my check engine light came on, the temperature gauge stopped working, the A/C stopped blowing cold air, and the fans started running on high. This time the code was showing that it was the thermostat not working. I got that replaced. Everything was fine. A few days later the same thing happened again. This time I just disconnected/reconnected the battery and everything started working fine. That was a few days ago and today the same thing happened again only this time the temperature gauge is at 0 even after disconnecting/reconnecting the battery and A/C is not blowing cold air. Could somebody please help me out?

It sounds like the engine coolant temperature sensor is out again causing the problems you are describing. When it was replaced did you use OEM parts? Here is a guide to show you what you are in for when doing the job and the location of the sensor for your car:

https://www.2carpros.com/articles/coolant-temperature-sensor-cts-replacement

Check out the diagrams (below). Please let us know what happens.

Cheers, Ken

Yesterday the temperature gauge on my daughters car would not move. It never came off the cold setting then the service engine light came on. When she called Onstar the told her there was a engine coolant problem. I met her and checked the reservoir and there was antifreeze in it. The car was not overheating so I gave her my vehicle and drove hers home. I stopped at an auto parts store and purchased a temperature sending unit and changed it when I got home. Started the car and still had engine light and no movement on the gauge. Also, the AC will not cycle. I then disconnected the battery to reset the computer and re connected. Started the car and all was fine. Gauge worked, AC worked so she took the car home. On her way home she called and stated gauge did not come off cold again but no service engine light. Too cool out so she could not tell if AC was working. Could it now be a stuck open thermostat or would you be thinking a computer issue?

i would have really like to have know the exact code numbers that you had before you changed the sensor. Anyway you can call Onstar and ask them what the exact code numbers are they would be a letter and and four digit number. That would really help because some cars are known for certain problems setting certain codes.

Hello, the code was P0128 according to my daughter.

That code is for a bad thermostat not a bad coolant temperature sensor. i would replace the thermostat it is bad. That is where i start. That is a common code for a bad thermostat.

That would explain the low temperature gauge reading. the thermostat is allowing coolant to circulate at way too low of coolant temperature.

That is not the case as the temperature gauge works fine showing about 190 degrees. After the battery is disconnected and reconnected. After running the car for maybe an hour or so then all of the sudden the gauge goes right to 0. It does not decline slowly. Then after about five minutes the check engine light goes on, the fans run continuously and the AC will not cycle. I can reset the computer and it will be fine for a while then it does it again. Sometimes after twenty minutes or less.

All the GM cars and other cars i have seen set the p0128 . On some cars if the coolant is too cold the AC will disabled. So the only code setting is the p0128? If i had the car i would replace the thermostat and go from there. i would also when the temperature gauge was reading low i would see what the temperature on the scan tool live engine data read to actual temperature with a infrared temperature gun. I have seen the PCM go bad and cause this

The coolant temperature sensor you replaced might have a issue who knows. i have seen some weird things happen with aftermarket parts quality compatibility and just plain bad right out of the box or bad shortly after installed.

The engine cooling fan would stay on if the coolant temperature sensor was reading back to the computer like minus forty degrees due to a circuit issue or coolant temperature sensor issue. That is why i am asking if you only have a p0128 still? There are other codes that set for coolant temperature sensor circuit issues like coolant temperature reading too or too low.

I am replacing the thermostat in a couple of hours and I will see if it solves the problem and let you know.

So what codes are setting now? You said the check engine light is coming on again?The thermostat is not going to cause the radiator fan to run constantly. If you do not live in California you can go to a auto parts store and have the codes scanned for free. I need the exact code numbers not code descriptions.

I had to get a rebuilt PCM cost me $187.00 all set. All fixed! Thanks for your help I love this site :)

Nice work, we are here to help, please use 2CarPros anytime.

Cheers

I have had this same problem with my G6. I have had the thermostat and sensor replaced. Worked for awhile then the gauge will work and stop and work and stop. My check engine light comes on like every twenty times it does not work.

Was this ever fixed and completely done with? I have been dealing with this for months!

The coolant gage quit working. I have to remove the hot wire off of the battery to get the coolant temperature to keep working. After a while the coolant temperature quits working. than the cooling fans come on and never shut off at all. Do I have to replace the coolant temperature gage. Or do I have to replace coolant gage sensor to make the gage able to work again. Or could it be a problem with a model or just a dirty sensor wire connection?

Inspect and test the coolant temperature sensor.

Also wanting to know if was ever fixed. just bought 2008 G6 Friday and Sunday same thing going on check engine and gauge will not move. Taking back to car lot today to make them fix it but probably they have already fixed this issue when they got the catalytic converter and it is starting again. I love the car but do not want to pay a note if this will continue to happen.

I would start with the sensor itself.

Yes, it was fixed with replacing the thermostat and the temperature sensor.

Okay, reading through all of this and making my own conclusions. I have a 2008 G6 with 72,000 miles bought it three years ago with no issues until just recently. While driving down the highway my temp gauge dropped from a normal 220-225 down to 0 degrees. I drove it home about twenty minutes and no check engine light came on. Went to the auto store and purchased a temp sensor (ECT), disconnected my battery, and all was well. That was three weeks ago. Just yesterday the same thing happened except my check engine light came on. Now again I am only going to assume that the code is going to be that P0128. But I am going to purchase a thermostat, replace it, and see what happens anyway. If for any reason that does not work, what else should I do?

My 2006 G6 temperature gauge jumps up and down and makes the car lag while driving. Code that came up is P1258. Changed thermostat and temperature sensor already. And it is not overheating.

Hello,

You need to hook it up to a scanner that will should you the sensor temperature and if it is okay from the computers point of view the computer is probably bad and needs replacement. Check continuity from the sensor to the PCM

Please let us know what happens. I am curious. ;)

Cheers, Ken

We replaced the thermostat again and also put on a new water pump. Car runs like it should now. Thank you.

Nice work, we are here to help, please use 2CarPros anytime.

Cheers, Ken