Need some advice P0128

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VAhlene

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I keep getting code P0128.

First I checked coolant level and it was fine.

Next, that weekend I drained coolant and replaced thermostat (Napa)(G-05 Coolant) reset and check engine light went away for about a week.

Light comes back on about a week later so I replace the Coolant temp sensor and reset, light has now come back on when wife was driving home from work. I am kinda lost as to what I should do at this point as was hoping someone might have some info. My heater seems to work fine and the temp gauge is sitting where it always has. Is there anyway to tell if the new thermostat I put in is working? Can you tell when it opens up?
 

Sephiroth

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Couldn't the IAT sensor be at fault since it is taken in to account when testing for the P0128?
 

VAhlene

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T stat is from Napa. I haven't read anything about the IAT sensor has that been known to cause this code as well?
 

Sephiroth

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take this with a grain of salt but when testing for the code, it is based on how long it takes the engine to reach operating temperature so it uses the readings from the coolant temperature, (which you said seems to be fine) and the air intake temperature. if the air is cold then it should take longer to get to operating temp, so if the IAT is telling it that the temp is hot and still take too long to warm up the code will pop up, thinking that the thermostat is stuck open. I am sure that someone will correct me if I am wrong.
 

LibertyTC

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VAhlene

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Yes the Napa tstat was 195 degrees and looked identicle to the one I removed. Sensor brand is duralast as AutoZone was only local store who had it in stock at the time.
 

VAhlene

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Here is a good read. see post 3 by TJKJ for time mentions.
Link: http://www.jeepkj.com/forum/f196/thermostat-specifications-jeep-liberty-kj-3-7-a-56107/
The OEM thermostat has an air bleed hole and as above & has to be installed as per diagrams.
The NAPA thermostat that was installed can you confirm it was a 195 degree?
Also if you changed the coolant temps sensor, what brand did u use?

Thanks for the link.... The OEM Mopar thermostat is actually cheaper than the Napa one, I paid $22 vs the OEM $16 off Amazon, for $16 to me it's worth it to swap it out, I will order, install and see what happens.

Also one more question, is anything going to get hurt in the meantime if my wife still drives it? She commutes to work about 50 miles round trip each day.
 

tjkj2002

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Thanks for the link.... The OEM Mopar thermostat is actually cheaper than the Napa one, I paid $22 vs the OEM $16 off Amazon, for $16 to me it's worth it to swap it out, I will order, install and see what happens.

Also one more question, is anything going to get hurt in the meantime if my wife still drives it? She commutes to work about 50 miles round trip each day.
Just the lack of heat(or taking forever to put out hot air) and taking forever if ever to go into open loop resulting in real bad gas mileage.
 

VAhlene

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I guess what makes me think it is sensor related is, the Jeep warms up just fine like it always has and the heat works just like it always has, seems to run fine and no major loss of MPG actually just got a little over 300 miles out of a full tank. I will try an OEM thermostat but I honestly don't think that's it.
 

LibertyTC

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If you changed the coolant temps sensor, what brand did u use?
 

tjkj2002

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I guess what makes me think it is sensor related is, the Jeep warms up just fine like it always has and the heat works just like it always has, seems to run fine and no major loss of MPG actually just got a little over 300 miles out of a full tank. I will try an OEM thermostat but I honestly don't think that's it.
It could be the ECT sensor but highly doubt it,a scanner and laser temp gun can find that out in less then 30 seconds.I have dealt with P0128 hundreds of times at work and a full 100% have been bad t-stat's and that is from all makes and models with the stupid GM 4.2 I6 being the most common(most r******d t-stat design ever).For that code to set the PCM looks at the start up temp and times how long it takes to reach 210+ degrees taking into account engine load(idling or driving).Will only time the warm up when the start up temp in under 80 degrees(if I remember correctly,could be 120 degrees as it varies by make).
 

tjkj2002

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As I stated before it is duralast, as autozone was the only parts store in town that had one in stock.
Yeah I'd be replacing that,better to put the OE back in.


Got 4 neverlast alternators for the same car sitting at the shop that they don't want back.All are defective out of the box and they had the only ones in stock in town also.After paying me 15 hours putting defective alternators in the same car they finally waited 1 day for Napa to get one in and it worked.Oh and autozone paid the 15 hours of labor to the shop.
 

VAhlene

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Would one from Napa be alright? The brand of sensors they carry are Proformer, Echlin or Delphi. Or should I just go straight to the dealer?

** Edit **
Just read that OE brand is Delphi... is this correct?
 

VAhlene

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Put an OEM thermostat in this morning so far no check engine light, we will see though as last time it took a couple days for it to come back on. I hope this fixes the whole issue as if it does not, I'm going to have to take it in somewhere to get a better diagnosis of what's going on.
 

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