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I have a 2013 JK with about 61k on it. About 2 weeks ago, the check engine light came on . Got the code and it was P 0128 which is thermostat. But it has not exhibited any signs. Was about to replace the thermostat, refilled the overflow and about 2 days later the light went out. All of a sudden a couple days ago, it came back again...now when u do the key test for the code it goes - - - - - - - - then it says -done- . Im perplexed at this point....any help would be appreciated. Thanks
 

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I have a 2012 and got a light for "thermostat rationality error" which apparently translates to the computer though the thermostat wasn't opening fast enough. No symptoms as far as overheating, etc. The code was read with the reader at the dealership. Replaced under MaxCare warranty.
 

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Ya that’s what im thinking, that it’s opening too fast, the spring is probably worn. Probably just going to replace it and see what happens.
 

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I'd replace the thermostat with an OE thermostat, clear the codes and see what happens. That's pretty much what the dealership will do. If under warranty have them do it.
 

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Mine (2012) just started doing this too. I've reset it. It's come back, but also seems to reset itself. From the FIXD app (code reader). Ironically.. here it says "severe", but when you click into troubleshooting, but when you clock for detail diagnostics it says: "Code P0128 Severity – Low"

Here's what the overview says:

What does that mean?

The powertrain control module (PCM) tracks how long it takes for the engine of a vehicle to reach and maintain the correct operating temperature. When the correct engine operating temperature is reached, the powertrain control module orders the fuel system to enter “closed loop” where readings from the oxygen sensors are used to maintain the efficient air to fuel ratio of 14.7:1. Most PCMs mandate that the engine coolant temperature sensor record temperatures above 160º F within 15 minutes of the engine starting. Additionally, once the 160º F threshold is achieved, the recorded engine temperature must not fall below 160º F during operation. The PCM will record if either of these two criteria are not met. If either fault is recorded again on the next engine startup, code P0128 is triggered.


How bad is it?

Severe - A stuck open thermostat is most likely the problem, however thermostats are usually designed so when they fail they are stuck open allowing full flow of the coolant protecting your engine. Check your coolant level first to make sure it is full before continuing to operate the vehicle. Overheating your engine will result in engine failure
 
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