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Discussion Starter #1
CEL p0128 code 'coolant temp below thermostat regulating temp'

This would show up on cold days

So I thought my IAT may be bad, so for $20 I changed that, but I am still getting the CEL and , in addition, code p0113 'IAT sensor 1 circuit high input'

Any ideas?
 

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Your thermostat is probably stuck open. Looks also like the sensor you put in cause another fault. I would put the old one back to see if it clears.
 

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Check the thermostat it's probably stuck open. If it is replace it with an OE thermostat, and clear the code.
 

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Discussion Starter #5
To check the thermostat: with the engine cold, open the radiator cap , start engine , and, if coolant is flowing, then the thermostat is stuck open?
 

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Did you buy the Jeep used ? if so, it might be possible the previous owner might have replaced the factory T with a lower temp one, if that was the case, you will need to replace it with a factory thermostat.
 

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To check the thermostat: with the engine cold, open the radiator cap , start engine , and, if coolant is flowing, then the thermostat is stuck open?
I am sorry, you can tell if the thermostat is open by checking "if coolant is flowing"? What does that even mean? Only way to really check it is to pull it and look to see if it is open...
 

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Discussion Starter #9
I thought you could open the radiator cap and start the engine. If you see the coolant moving immediately, then the thermostat would be stuck open - coolant flowing before the engine heats up enough to open the thermostat.

?
 

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Discussion Starter #11
OK, got the thermostat & housing (FYI:It's one piece and lists for about $50). When I did a search for thermostat , I found all these p0128 heads that I somehow missed when I thought it was the IAT . oh well, I'll try to get this done this weekend.
 

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OK, got the thermostat & housing (FYI:It's one piece and lists for about $50). When I did a search for thermostat , I found all these p0128 heads that I somehow missed when I thought it was the IAT . oh well, I'll try to get this done this weekend.
Let us know how it turns out. Make sure you use the right coolant to refill. Also keep an eye on you overflow tank as you work the air out of your cooling system make sure it is topped off regularly.
 

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could be the water temp sensor as well. wifes 3.7L on her Jeep commander had the same problem until we replaced it.
 

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Discussion Starter #14
I didn't do anything to it this weekend, just no time. Then it went off by itself for a few days, but now its back on. Don't know if its because I had a longer drive, or the outside air warmed up a bit.
 

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I didn't do anything to it this weekend, just no time. Then it went off by itself for a few days, but now its back on. Don't know if its because I had a longer drive, or the outside air warmed up a bit.
From this description I would say your thermostat is definitely stuck open. I think the CEL will clear after some number of restarts. If those are warm restarts it won't throw the code. But the longer is sits and cools the longer it takes to warm back up, then you get the CEL. There was a thread that explained how it figures out that the coolant is warming up quick enough. What year JK?
 

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I currently have the same issue on my 2011 Chrysler Town & Country (my OTHER car). Same code and temperature would not get above 180F or so and that was driving around down. When I would get on the highway it would drop to 150 to 160F. My conclusion was a bad tstat that is stuck open, so I have a mopar on order (got on ebay for $13.50 shipped). Should be here next week and hope to get it installed one night next week. I will let you guys know if it fixed my problem.
 

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Discussion Starter #18
I didn't get to this on new years day like i should have, but now it's now colder and the check engine light has turned off and I no longer am getting any codes.
 

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I am currently experiencing the same thing on my 12 JKU. The research I did pointed to the coolant temp sensor. I purchased one and replaced it then cleared the code. It was fine for 2 weeks but came back yesterday. I cleared it again and it stayed off even with the temps dropping into the low 30's.

I guess I need to get back on the rock auto site and order the thermostat assembly as the next step.
 

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Check eBay also. I found a Mopar t'stat for $13.50 shipped.

I am currently experiencing the same thing on my 12 JKU. The research I did pointed to the coolant temp sensor. I purchased one and replaced it then cleared the code. It was fine for 2 weeks but came back yesterday. I cleared it again and it stayed off even with the temps dropping into the low 30's.

I guess I need to get back on the rock auto site and order the thermostat assembly as the next step.
 
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