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Just got this code. Winter finally hit N.Texas. I watched the coolant temp on the evic and it looks like the thermostat is stuck open since the temps aren't fluctuating like they should when they hit the thermostat's initial opening temp. Only 23800 miles on a '14.
 

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It's the thermostat, I just bought a 2013 jk and my light came on with same code. Took to dealership since I bought extended warranty and they replaced thermostat.
 

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Heads up that my 2012 JKU threw a P0128 and it was just because my coolant level had dropped below the min line. The colder weather was enough to trigger it. Once I added some coolant the code cleared and the CEL went away after about 3 drive cycles (about 3 days later). Not sure if any of your issues are as simple as this but I'd check your coolant level after the Jeep has sat cold overnight and make sure the overflow box level is above the min mark.
 

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To those that have had this problem - does your dash temp gauge read low?
yes. My evic shows that the thermostat is fully open at around 174 degrees, instead of around 200 plus or minus 3 degrees. After a long drive, it would get up to around 188 degrees.
 

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For reference, here's some data from my drive home from work today with a correctly functioning cooling system and thermostat. Even with ambient temps at 24*F, I still reach 195*F coolant temp in less than 6 minutes, then settles into fluctuating between 194-199*F.

 

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And a similar drive in the summer:

 

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Hoat
 

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I kept getting the code P0128 so I ordered and installed a new thermostat and housing and I have not had the check engine light come on again. That was about 4 months ago. The code was popping up every morning for a few weeks. I had never replaced a thermostat before and this was very easy.
 
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