Chrysler Repair: 05 T & C: inadequate heat/code P0128, coolant temp, engine coolant

Question
My wife has been complaining that her van isn't warming up and I just said that it's a very cold winter and she only drives about 4 minutes to work so it wouldn't warm up that much anyway. We left for a few days as a family in the van and she was right! For over 3 hours of highway driving the temp never got off of the "C" and if it did it was barely! During that trip my engine light came on and my code reader gave the code P0128. I looked it up and most of what I found was low coolant. My coolant is clean and full! What should I check next? Is there a sensor thats bad, if so where would I find it? HELP, the frozen family!

Answer
Hi Charles,
It appears to me that you have an engine coolant thermostat that is stuck open so that the coolant temperature is not coming up to the proper operating level (in the range of 200F). The lack of heat/low reading on the gauge/code 0128 are all consistent with that explanation. You might want to do one thing before you change it out: after driving a few miles touch the two hoses that go through the fire wall at the rear of the engine compartment and note whether they are similar in temperature. They should be if you have coolant flowing through the heater core. If one it noticably hotter than the other then your heat problem may be due to obstructed coolant flow through core. But I think you just are not getting up to operating temp becuase the thermostat is not working properly.
The 0128 can be caused by low coolant, but you state that is not the case. There are a number of other causes including a faulty thermostat that you didn't see listed in your reference. Basically the code says that the coolant temp is not what the engine computer believes it should be based on the operationg characteristics that it has been put to since starting the engine. It also can be caused by extremely cold weather so that may be why you have it logged. So setting that aside we are back to the classic reason for no heat and low temp gauge reading: a stuck open thermostat. If you want to replace it yourself then tell me which engine (L) you have and I can tell you any step you should follow in doing the job.
Roland
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