P0128 Thermo failure

[JPNinPA]

Short version
P0128 Thermostat Rationality
P0128 JEEP
Description
Indicates that the thermostat monitor has not achieved the required engine operating temperature within a specified amount of time after starting the engine.
Symptoms- CEL
Possible causes
- Insufficient warm up time
- Low engine coolant level
- Leaking or stuck open thermostat
- Faulty MAP sensor
- Engine coolant temperature sensor harness is open or shorted
- Engine coolant temperature sensor circuit poor electrical connection
- Faulty engine coolant temperature sensor When is the code detected?
Coolant temperature does not reach predicted engine coolant temperature within specific time
Tech notes
If cooling system is not leaking and the fluid level is OK, replacing the thermostat usually takes care of the problem.
This one was only 6 mo old

 

[steady.foot]

My guess is, A faulty thermostat.

 

[JPNinPA]

Well replaced that Stant stat with a Stant superstat. Well as soon as the weather turned cold the P0128 came back. I noticed it again took most of 5 miles of driving to heat the engine up to 210. Not even 195.
So I replaced the new superstat with a NAPA We'll see how this does. I would have stayed with Stant if I could have gotten the OEM Stand locally. I believe the reason it all came back is the superstat opens gradually and early. This may be o.k. in warm weather but in the cold temps >40* it did allow the engine to heat quickly. My MPG also suffered.
I also checked the temp sending unit. cold 10.4K ohms and app 160* it read ~1K ohms. I need to take it at 210 and be sure it's less than 200 ohms. Or I'll be replacing the temp sending unit. This is an order and no return policy part.

 

[JPNinPA]

So it's been a few days now and no more CEL. Also another forum member listed the temp sending units expected resistances at 3 different temps. So it looks like the sending unit is good. At 212 it should be 640-720 Ohms.