So I replaced the thermostat and the ect sensor. It's still running the fans all the time. It puts out heat. Just not really hot heat. So what else could it be? I have a 2000 Monte Carlo SS. 3.8l. I'm lost. Only thing I can think of is a wire has gone bad. Any help please.

 

Check the cooling fan circuit. You may have a bad relay or (as you suggest) a bad wire in the harness. Once you get the fan situation under control I have a feeling your P0128 problem will go away. Even though it indicates a problem with either the thermostat or ect, you have to keep in mind the code will also be triggered if coolant temp is too low. With the fans constantly running from start up the coolant may not be reaching operating temp within the allotted time before the PCM sets the code.