2007 base model showing PO128 code. Have replaced thermostat twice, no change. Engine does not run hot. Temp. needle gage stays flat while dig. gage will show 99-102. Sometimes after running for few minutes dig. gage temp will drop to 95-97. After an hour or so of driving, will get a message reading Hot engine, A/C off.
Could this be a temp sensor and if so, where is it located.

Thanks, Steve

 

P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature ECM

So either you have a 160* stat that is opening way before even 160* and the temp of the motor is not coming up fast enough to begin with, or you still have air in the system that is causing problems instead.

To get the air out of the system if your running a normal stat, you need to put a vacuum on the reserve tank to draw the air out the block passages that way.

As for the over heat problem, could be air in the system, or could be that the radiator fan is not coming on.
To check the radiator fan, start the car, turn the A/C on, and the radiator fan should be running with the A/C on, even with the motor idling.

If the fan does not come on, then start with disconnect the radiator fan connector back passenger side of the radiator, and make sure that the connector pins are not burnt.

The laser red dot is on the connector,
Attachment 48334002

And you don't want to see this when you check its connections,




If the connector is not the problem, then turn the motor off, reach in and turn the radiator fan blades by hand. If the blades are hard to turn by hand, then may be the raditor fan motor that is burnt out, the PWM, or maybe a fuse back in the engine fuse block.

Lastly, if the car was tuned, then have your tuner go back in and double check the fan tables to make sure they are are still within range/flashed in correctly.

This is the stock tables, so with luck, was not flashed well beyond these to have the fan just begin over driven way too much to burn the motor out quickly isntead.
Attachment 48335485
And again, A/C will over ride these tables to turn the radiator fan on when ever the A/C is turned on. Hence A/C heat exchange coil is in front of the radiator, so ECM tunes the the fan on to draw air through the heat exchanger coil, not matter what temp the motor is running when the A/C is turned on.