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Old 02-17-2013, 09:09 AM   #1
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Code PO128 Coolant temperature

This engine light keep coming on 2 days after I clear it. I have replaced the coolant sensor, thermostat 2 times and the serpentine belt just for the heck of it and this engine light keeps coming back on. Any answers to why? How about the engine coolant itself, would it matter.

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Old 02-17-2013, 02:18 PM   #2
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Make sure you are using the proper thermostat (195º) I think. The code means it is not reaching proper temp or not fast enough. The IAT sensor could be bad especially if you are in a cold climate. Read this link, it may help.

OBD-II Trouble Code: P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

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Old 02-17-2013, 04:43 PM   #3
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Thankyou for that link showing the reasons why the po128 code comes on. I will be checking all the parts. It is 20 degrees here and the jeep was fine all summer and fall till it got real cold, then the engine light came on. Thanks again. Will let you know when I find the fix.
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Old 02-17-2013, 05:45 PM   #4
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The way I understand it, the IAT lets the computer know (among other things) how cold it is so it can adjust how long the warm up should take. If it is not sensing correctly it expects a faster operating temp & when (if) that doesn't happen it throws the code. (?)

Maybe on warmer days or on later-in-the day start ups it won't throw the code. That might point to a bad IAT sensor. Although there may be a certain # of failure cycles before throwing the code, I'm not sure. Again, make sure you have the correct thermostat.
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