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

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

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

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

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.

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

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

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.

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.