Coolant temp below thermostat regulating temp

P0128 is an insufficient coolant temp code. Which means the coolant temp is not coming up to full operating temperature fast enough or not at all. The fix for this is to replace the thermostat.

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Yes , having the vehicle checked for DTC'S - diagnostic trouble code's should always be the first step in diagnostic process when any warning lamps are lit on the instrument cluster . Would be best to have someone with a professional type scanner , that can view engine sensor data . The engine coolant temp. sensor is a input to the PCM - engine computer . Can see what the actual temp. of the engine is . GM had bad stepper motors inside the instrument cluster's 2003 to 2006 , these stepper motors control the gauge needles . Testing what the real temp is an not going by temp gauge . harbor freight tools sells a tool under $30 bucks that can check engine temp . Infrared Laser Thermometer .. Point it at the thermostat housing . Can use it to tell what temp. the thermostat open . But , testing is the key ! Not guessing .

Is your fan working? Did you check the HVAC fuse? In the summer is the Air Conditioning clutch even turning on? Please rate my answer so I get my points for it.

On the 3.8 liter the coolant temp sensor is right below the the thermostat in the intake manifold on the drivers side of the engine.

The thermostat is under the housing that is bolted to the intake plenum that the upper hose is attached to.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Well, I can't give you any specific information regarding your vehicle because you forgot to mention what kind of vehicle you have. (This is kinda important if you really want some help figuring out what is wrong with your car)

The "Generic OBD" of code P0128 is "Coolant Temperature Below Thermostat Regulating Temperature"

The most likely cause is a stuck-open thermostat, however, I have also seen defective Engine Coolant Temperature (ECT) sensors also cause this code.

Yes that is what the code means. GM usually uses two, one that makes the gauge work and one that tells the ECM what the engine temp. is. You need the one for the computer.

What your describing is either a sticking thermostat, a low coolant condition or a blown cylinder head gasket.The code is for coolant temp fault.Not always the temp sensor but it is located next to the thermostat housing.

P0128 - Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

P0410 - Secondary Air Injection System Malfunction
(has to do with exhaust system)

Note: There is a technical service bulletin (TSB) regarding code P0410 for a number of GM models. The TSB calls for the replacement of the AIR pump and the installation of a new secondary injection hose.

Your thermostat will not affect the heat in your car. This is controlled by the heater core which is pulled from coolant from the engine where the coolant will be the hottest. Your thermostat opens and allows the coolant from the engine to circulate into the radiator and the coolant from the radiator flow into the engine. A thermostat is cheap and easy to replace. The sensor should be a fairly inexpensive and easy replacement as well. I would change out the sensor first because bad thermostat would create an overheating problem.

check your temperature sending unit and be sure it is working properly and then check to see that your thermostat is working properly if you are getting heat inside of the vehicle from the heater then this would be a good indication that your thermostat is working properly. also check your coolant level is correct.
code P0128 is coolant thermostat ( coolant temp below thermostat regulating temperature )