Replaced the coolant temp sensor and thermostat. It was done 2x with new parts due to leak on first install which is now gone. Reason for change was check engine light and code p0128c. Code is able to be read and cleared but comes back. Sometimes it takes 20min and 1 trip. sometimes hours and few startups and shut downs.

Read that a battery can throw a false p0128 but battery sits at 12.5 to 12.6 not running. I did notice that when the code came on last time, I saw the temp gauge fluctuate as I was trying to monitor it. Car starts runs and drives fine.

Not sure if the outside air temp sensor is related but air temp reading is off by 10 degrees high or so but has been that way as long as I remember(not sure how to get to it and if it can be cleaned, relocated or recalibrated.)

You will need to post details of the code description so we can go further.

The fault criteria for P0128 are pretty simple. If the engine temp doesn't rise according to the "map" a fault is set........of course the "map" has many variables

Is the auxiliary fan running all the time??? Is the outside temp sensor accurate??? I once found a thermostat installed backwards on an M271 that caused P0128.

My point was that if the original poster ( and many of the others that have posted code # questions ) researched the full code description / conditions that set it, they would have a better understanding of what is going on.

What does the scan tool data show for coolant temp and coolant temp voltage? I say to check voltage ( even with a meter at the sensor / computer ) because on a few OBD one systems I've had sensor wiring fail and the computer substitute a believable value for coolant temp but the sensor voltage was incorrect.

I don't know if the posters OBD two system would sub a value or not.

Can engine coolant temp still be brought up on the HVAC display like the older cars?