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post #1 of 13 (permalink) Old 10-15-2012, 11:52 AM Thread Starter
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P0128

Saturday morning, we had our first cold snap in No.Va. and I usually head 45 minutes down the road to fly RC airplanes just south of Fredricksburg. So, 5 minutes after I get on 95 south I see the check engine light on the dash, but a check of the gauges and listen to the engine provides no clues.

On returning to the homestead, I did the 3 click DTC check and see that 128 is relevant to the thermostate but the wording is really weird. (rationality error has been detected for the thermostat) Anyhow, I noted that the coolant level was below the Blue Add line, so I picked up a gallon of premium antifreez and raised the level to the full mark. I also disconnected the power from the battery terminal, and sure enough the light went out.

So my question is, did the Jeep folks just poorly word the code?
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post #2 of 13 (permalink) Old 10-15-2012, 01:10 PM
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I hope you used the correct coolant. See this thread---> https://www.jeepcommander.com/forums/showthread.php?t=9422&page=7

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Last edited by jeep5253; 10-16-2012 at 11:17 PM.
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post #3 of 13 (permalink) Old 10-16-2012, 11:03 PM
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My first guess would be a bad thermostat. They are cheap and I would start there.
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post #4 of 13 (permalink) Old 10-18-2012, 05:36 AM Thread Starter
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For others following this thread... instead of being cryptic. the correct Antifreeze is Orange in color...

However, getting back to the nub of the question. Since topping up, and re-setting the error code there's been no New P0128 thrown. The "Bozo" that came up with Rationality error could have come up with some better code. MS Word shows rationality as: Level-headedness. shrewdness, wisdom Sagacity, prudence, at the bottom is logic and unity and reason. How about Thermostate "effed up position"?
If that's what it really was. As I indicated, the morning was the first Brisk chill, and the coolant was low. I'm wondering if they use that code for multiple error possibilities, like low coolant level detected, as well as engine thermostat range error.


KKKKFL
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post #5 of 13 (permalink) Old 10-18-2012, 06:19 AM
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The "bozo" knew what he was doing.

Definition of Rational: agreeable to reason; reasonable; sensible
(ie: MS Word synonyms isn't exactly the best source for a "definition")

Theory of Operation for P0128:
Quote:
The PCM predicts what the engine coolant temperature should be, based on the engine coolant temperature at start-up, ambient temperature and how the vehicle is subsequently driven. The predicted engine coolant temperature is compared to the Engine Coolant Temperature Sensor reading. The error between the two is calculated and integrated with respect to time. When the Thermostat diagnostic runs, the integrated error is compared to a calibrated threshold and pass/fail is determined. Separate pass and fail thresholds are used in order to improve accuracy of the diagnostic.
Thus, the code is anything BUT a "yo, your thermostat is stuck" code. As a matter of fact a stuck thermostat could throw off a bunch of different codes. There is not a specific DTC for every possible failure mode in the vehicle. That's why there are auto techs; for any given DTC there's usually a list of 5 to 10 possible causes.

For this particular code there are a bunch of possible causes.... low coolant, stuck thermostat, bad temp sensor, wiring faults, PCM fault, etc.

-Matt

Last edited by Matt_; 10-18-2012 at 06:28 AM.
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post #6 of 13 (permalink) Old 10-18-2012, 12:38 PM
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Quote:
Originally Posted by BigBlue View Post
For others following this thread... instead of being cryptic. the correct Antifreeze is Orange in color...
There is more to the correct coolant than just the color, which is why I gave you the link to the thread about it.

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post #7 of 13 (permalink) Old 10-18-2012, 04:35 PM
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There are a bunch of thing the p0128 could be and low coolant is one of them but I haven't seen that happen unless you were really really low. There are a number of sensors it could be. However, I have heard that when cold and you just take off driving for some reason it won't detect warming up fast enough and throws that code. Kind of like a stuck open thermostat.

It is important to use HOAT coolant though. Regular anti-freeze isn't as nice to aluminum. The HOAT protects much better against corrosion of the aluminum.
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post #8 of 13 (permalink) Old 11-10-2014, 06:26 AM
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The correct anti-freeze is the one stipulated in the Owner's Manual, it is Chrysler HOAT and the aftermarket eqaivalent is Zerex G-05.

The color is just dye, and the other Orange Anti-Freeze is GM DexCool which is a disastrously wrong anti-freeze to use.

Most of the "Universal" and Generic anti-freezes are now a derivative of GM DexCool, almost all anti-freezes protect aluminum well, but GM Dexcool has additives that are NOT compatible with parts in other cooling systems NOT designed for it and has a whole set of drawbacks compared to other anti-freezes.

That is why it is important to get the right anti-freeze, by the specification, NOT the color.

Unless you mixed different types of anti-freeze to cause a cooling system problem or did NOT mix it the correct ratio with water, I doubt the anti-freeze is your problem. P0128 means the vehicle is NOT warming up fast enough. A different anti-freeze is still going to conduct heat close enough to the others that is unlikely the problem.

More likely its the thermostat.


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post #9 of 13 (permalink) Old 11-10-2014, 07:57 AM
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Quote:
Originally Posted by Matt_ View Post
The "bozo" knew what he was doing.

Definition of Rational: agreeable to reason; reasonable; sensible
(ie: MS Word synonyms isn't exactly the best source for a "definition")

Theory of Operation for P0128:


Thus, the code is anything BUT a "yo, your thermostat is stuck" code. As a matter of fact a stuck thermostat could throw off a bunch of different codes. There is not a specific DTC for every possible failure mode in the vehicle. That's why there are auto techs; for any given DTC there's usually a list of 5 to 10 possible causes.

For this particular code there are a bunch of possible causes.... low coolant, stuck thermostat, bad temp sensor, wiring faults, PCM fault, etc.
I got P0128 this morning, and yes it was considerably cooler than the last time I drove the vehicle, like a more than a 20F change.

Since the PCM is comparing the OAT with the Coolant temperature to see if the vehicle is warming up within a expected amount of time. I wonder if the P0128 can be erroronously thrown by the OAT sensor calibration? Personally I've noticed the OAT on my '10 Commander always shows the last temperature read from the last time I drove the vehicle and parked it, until the vehicle has started moving for a few minutes and then it adjusts to the actual temp. I remember posts about the OAT calibration, and some of the alogrithms it goes through because of the difficulty of reading OAT on a vehicle that sits in the sun.

So my Commander certainly seemed to take the normal time to fully warm up this morning in 36F weather, but it threw the code P0128. I didn't look at the OAT when I started out this morning, but I wouldn't be surprised if it read the same 57F from when I drove it last night. That it didn't adjust back to the actually 36F until a few minutes into my commute, when I started moving fast enough to get enough airflow into the grill and over the OAT Sensor to give a reliable figure.

Probably won't happen again, unless we have a warm snap, because future OAT readings, which will start out at the last OAT reading, will be much closer to the actual temp.


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post #10 of 13 (permalink) Old 11-11-2014, 01:27 PM
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Mongo:
I've been getting the P0128 code recently as well. For me, it has happened 3 times int he last 30 days. I was more concerned about it until I read you post, and specifically the idea that it stores "last night's" OAT to use a comparative.
Where I'm at, it has been 65-70d in the evenings when I park it, and 30-25d in the morning when I go out. That's what we call Fall around here.

Since the vehicle seems to warm-up correctly and I see no other abnormalities in the way it runs, I might assume this is what is happening?
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