I have 4x4 slt v8 4.7l 2001 dakota that has had a check engine light on since I got it. The code is p0128 I believe (coolant below thermostat regulating temp). I replaced the thermostat when I first got it and the reset the computer, light came on not even a week later. So I changed the coolant temp sensor, reset the computer and same thing, light came back not even a week later. Any ideas out there? Maybe the thermostat I replaced with isn't oem? Wiring? Any other thoughts or suggestions will be much appreciated. Thank you

 

The way I understand the P0128, the PCM looks at the engine running time, the temperature measurement from the "Intake Air Temperature Sensor" (IAT) and the "Coolent Temperature Sensor" (ECT) to determine if the Engine Temperature is at the desired operating Temperature. About the only thing left for you is the IAT Sensor.
Good luck.

 

Sounds like IAT sensor. It's the only thing left from what you described. I know if you have had any Plenum or Int. Maniflod issues, or even PCV issues recently, the IAT can get oil soaked and won't operate correctly. I would pull the IAT and be sure it's dry and free from any engine oil or carbon build up. Check your PCV also. Good luck!