So I put a new thermostat in my car over the weekend, and my water temp gauge finally stays at normal temp, but my CEL comes on after I run the A/C for a few minutes. I disconnected and reconnected the sensor, but it still comes on. What am I doing wrong?

 

What is the CEL on for? That would be a good clue as to what is going wrong.

 

pull the code, it could be something totally different than your thermostat. Does everything else seem to be running fine?

 

CEL didn't come on until after I changed my thermostat. I just don't get it everything else is running great. Water temp is perfect at all times. Fans run like they should and everything. I'm confused.

 

There is nothing to be confused about. Go get the CEL checked. Hell, it could be completely unrelated to the cooling system like an o2 sensor going bad.

 

I tested it and came up with a P0128 code. Keep in mind I have a brand new thermostat and there is plenty of new coolant in it. So it's gotta be electrical or a bad ECT sensor, right?

 

What temp Tstat did you install? Maybe the temp you got was different than stock, thus throwing a code since it could be opening too early or late based on OEM calibrations within the ECU. If not, then I would start to look elsewhere such as other sensors, or wiring.

 

Did you bleed the cooling system after replacing the thermostat? Any leaks?

 

Its called a FailSafe. As I explained before, if I run it for a few minutes until it gets warm without running my A/C, the CEL never comes on. Even if I turn my A/C on after it gets warm it still doesn't show up. BUT, after my car is cooled and I start it up again and turn my A/C on right away, the CEL shows up after about 2-3 minutes of running. I know this is exactly what it's doing bc I've tested it at least 5 times. And no, nothing is leaking.

 

I always thought fail safe was a type of Tstat, not brand (i.e. Stant), but I could be wrong. I always go with the OEM Tstats from the dealership so I dont have to worry about this type of stuff. Tstats can fail and remain stuck open right from the box.

ninja edit: What temperature Tstat did you install? 170, 180, 190?