Jeep Garage  - Jeep Forum banner
41 - 60 of 70 Posts

·
Registered
Joined
·
2 Posts
I have had this code come up once. I checked the code and then cleared the code. It has not returned. As a precaution I was thinking about changing the t'stat but could not find the info in the wk2jeep website. Could someone send it to me?
 

·
Registered
2020 Jeep Grand Cherokee Summit 3.6L 4x2
Joined
·
1,807 Posts
DUH! LOL :lol:
 

·
Registered
Joined
·
4 Posts
padgett, here's what I have to go on regarding condtions for the P0128 to set.

When monitored: With the engine running, ambient temperature between 8C(17.6F) & 50C(122F), startup coolant temperature less than 50C(122F) and average vehicle speed greater than 16KPH(10MPH), untill the vehicle untill coolant temperature reaches 85C(185F).

Set conditions: The PCM detects that the actual coolant temperature falls too far below the predicted engine coolant temperature and the predicted engine coolant temperature reaches the predicted target value before the actual coolant temperature reached the actual coolant temperature target value. Two trip fault. Three good trips to turn off MIL.

Possible causes: The top two are most common.
-Low coolant
-Thermostat open
-Temperature sensor
-Signal circuit open
-Signal ground circuit open
-Signal circuit shorted to ground
-Signal circuit shorted to the (K900), (G931) sensor ground circuit
-Powertrain control module (PCM)

Glad to know I'm not the only guy running into this same problem.

I've got a 2011 Grand Cherokee Laredo 3.6L. Check engine light came on a while back and found it was throwing a P0128 code. I replaced my T-stat a couple of weeks ago and cleared the code, however it came back on a couple days later. (Bonus: I tried to clear it again, but the code wouldn't clear - not sure why...is that considered a 'hard' fault?)

I drove it on a short trip this weekend and noticed my temp never went above 168 degrees, per the on-dash engine diagnostic information. I am guessing it's my ECT sensor that needs replaced next.

First question, what is normal operating temp variance for mine? Am I truly low? I would assume so, based on the fault I'm receiving, but wanted to confirm. Next, if I tackle the ECT replacement, is it an easy swap/what should I look out for/etc. and should I choose an OEM part or not? (Here is an O'Reilly part suggestion: BWD WT5205 - Engine Coolant Temperature Sensor | O'Reilly Auto Parts) I will try to clear the fault afterward and see if it trips again - if it does, do I need to see the dealer? (Just like doctors, I do NOT like going to see the dealer...$$.)

I have been literally pulling my hair out on this for a while, until I saw this post. It's getting cold here in the Midwest and I REALLY want my remote start back. :)

A huge thanks guys - Mike

_____________________________
'11 Grand Cherokee Laredo
'97 Cherokee Sport
 

·
Registered
2014 WK2 Overland
Joined
·
1,024 Posts
Glad to know I'm not the only guy running into this same problem.

I've got a 2011 Grand Cherokee Laredo 3.6L. Check engine light came on a while back and found it was throwing a P0128 code. I replaced my T-stat a couple of weeks ago and cleared the code, however it came back on a couple days later. (Bonus: I tried to clear it again, but the code wouldn't clear - not sure why...is that considered a 'hard' fault?)

I drove it on a short trip this weekend and noticed my temp never went above 168 degrees, per the on-dash engine diagnostic information. I am guessing it's my ECT sensor that needs replaced next.

First question, what is normal operating temp variance for mine? Am I truly low? I would assume so, based on the fault I'm receiving, but wanted to confirm. Next, if I tackle the ECT replacement, is it an easy swap/what should I look out for/etc. and should I choose an OEM part or not? (Here is an O'Reilly part suggestion: BWD WT5205 - Engine Coolant Temperature Sensor | O'Reilly Auto Parts) I will try to clear the fault afterward and see if it trips again - if it does, do I need to see the dealer? (Just like doctors, I do NOT like going to see the dealer...$$.)

I have been literally pulling my hair out on this for a while, until I saw this post. It's getting cold here in the Midwest and I REALLY want my remote start back. :)

A huge thanks guys - Mike
Before you go any further, verify your coolant level is full. 180-190ish is the norm for most newer vehicles. From the semi low reported temperature, it sounds alot like you have a bit of air trapped in the system. Generally if an ETC was to fail, the reported temperature would be extremely high or either extremely low. However, when dealing with electrical componets, strange things can and will happen.
 

·
Registered
Grand Cherokee
Joined
·
308 Posts
Discussion Starter · #47 ·
The t-stat switch fixed my code. Like somebody else said make sure it is bled correctly via the screw on to of t-stat housing then I guess try the sensor.

good luck
 

·
Registered
Joined
·
121 Posts
Just got the same code, will order a thermostat and hopefully fix the problem thanks for the bleeding procedure. says its a 203 degree stat. hot stuff!

BTW, I tried to clear the code and it keeps coming back on instantly, any idea why this happens? The coolant temp according to the dash is about 177 the gage is about a quarter.

Thanks
 

·
Registered
Joined
·
4 Posts
Just recently acquired a 2012 Jeep Grand Cherokee Overland. Not long after, I got the P0128 code. Following seeing this thread and replacing the thermostat, it appears to be fixed. Thanks for the help.
 

·
Registered
Joined
·
29 Posts
May not be correct but I have a '97 GC ZJ 4.0L 4x4. My mechanic told me not to reset the error code as the computer board requires 3 consecutive errors before the check engine light goes on and throws a code. He said to copy all error codes and as long as I'm aware of the problem and watching my gauges I should not reset the codes. It will allow any other codes to be triggered that can help to narrow down the problem further if a new code appears.
I'd verify this with a good Jeep Mechanic to be sure the advice is valid. The check engine light isn't going to make it worse, it's just warning you something is wrong so resetting the code without fixing the problem may be hiding other codes that have not appeared yet.
 

·
Registered
Joined
·
4 Posts
That's almost correct. My research indicated that it takes two P0128 faults to cause the check engine light to come on. Then, after the repair, it takes 3 good cycles from a cool engine to a warm engine in the allotted time to turn off the check engine light. Following my thermostat replacement and 3 good cycles, the check engine light turned off and has not returned. Resetting the code just prolongs the results.
 

·
Registered
Joined
·
1 Posts
didnt read this til i was done. let the bleeder valve bleed a few times before and after running. fluid level in the reservoir is good when the engine is hot and cold. how long does the check engine light take to turn off? its been almost 2 days, should i have patience or bleed the system like you said to?
 

·
Registered
Joined
·
4 Posts
I think mine took about 4 days, but I don't drive much. It takes a certain number of good cycles from cold to operating temperature to turn the light off.
 

·
Registered
Joined
·
332 Posts
didnt read this til i was done. let the bleeder valve bleed a few times before and after running. fluid level in the reservoir is good when the engine is hot and cold. how long does the check engine light take to turn off? its been almost 2 days, should i have patience or bleed the system like you said to?
Strange, I didn't get the check engine light again immediately after changing the thermostat.
 

·
Registered
Joined
·
4 Posts
That was my experience also. Then, the next morning it came back on. Further research gave me the information about the cycles needed to turn it off again. Anyway, whatever the number of cycles was, either 4 or 6, it went out exactly as planned and hasn't returned.
 

·
Registered
Joined
·
16 Posts
Subscribed
 

·
Registered
Joined
·
1,399 Posts
I just had this code read at AutoZone. P0128, THERMOSTAT RATIONALITY. 2011 WK2 with V6.

Here is my situation: I have MaxCare warranty from Chrysler and already have an appointment with dealer on May 7 (3 weeks from now) for "no heat coming from the vents on the driver side" and 2 recalls. This is the earliest appointment I could get with a loaner car (I must have a car to get to work)

Is it safe to drive (40-45 miles/day) for 3 weeks? Other options include going to my local mechanic and pay out of pocket (I'd rather not) or try to get an earlier appointment with this or another dealer.
 

·
Registered
Joined
·
2,135 Posts
I just had this code read at AutoZone. P0128, THERMOSTAT RATIONALITY. 2011 WK2 with V6.

Here is my situation: I have MaxCare warranty from Chrysler and already have an appointment with dealer on May 7 (3 weeks from now) for "no heat coming from the vents on the driver side" and 2 recalls. This is the earliest appointment I could get with a loaner car (I must have a car to get to work)

Is it safe to drive (40-45 miles/day) for 3 weeks? Other options include going to my local mechanic and pay out of pocket (I'd rather not) or try to get an earlier appointment with this or another dealer.
Hi JoeSchmoe007,
We appreciate you reaching out with this. We would like to offer our assistance while you are working with your dealer. Please send us a private message with your VIN and servicing so we can work on getting this escalated onto the proper teams for you.
Julie
Jeep Social Care Specialist
 

·
Registered
Grand Cherokee
Joined
·
4,654 Posts
I had this on a 2010 XK (same mechanically as WK), happened on very cold mornings below freezing. I tried a new thermostat and then a new Temp Sensor, both seemed to improve the situation, but it would come back. I also had an intermittent coolant leak, and noticed it happened when the coolant level dropped.

I can understand on how being low on coolant could cause overheating/undercooling; but how does it cause overcooling? Best I can figure, being low on coolant would result in the engine not warm-up evenly, some spots getting hotter while others stay cooler. And the temp sensor is near the top of the engine, where if you're low on coolant it's going to circulate less at the top of the engine, so that makes sense.

This was the 3.7L V6, different than the newer 3.6L V6. But my problem ended up being a minor, intermittent headgasket leak. I used some BlueDevil Headgasket Sealant, it really works. Not the "correct" fix for a headgasket leak, but it worked, I stop loosing coolant and I stopped getting the P0128 codes.
Mine came back on yesterday driving down the highway with it being 70 or so very humid and therefore A/C was on. I don't use A/C much so not sure if tied to that or no. I might try thermostat as well soon. Let me know how it goes. I have 88k on car.
Ummm, you know the AC being on dumps heat out of the Condenser into the Radiator, if anything, the AC being on should help the motor warm up sooner and will cause the engine to run hotter.

I stopped to help a woman stuck on the side of the road once. She had overheated her car so bad the radiator burst. She said the engine was running hot, so she turned on the AC to cool it off, and then things went really bad.

Oh really, you think?
P0128 sets when it takes longer than normal for the engine to reach operating temperature. Generally caused by a thermostat that is partially stuck open and/or low engine coolant.

If there is a problem with the coolant temperature sensor, there are several trouble codes that can set for that as well. ;)
This was added to OBDII for emission purposes. When the engine is cold it makes more pollution, so if the engine takes too long to warm up its sets a MIL to get the owner to repair it and keep the emissions down.

Now the vehicle was designed and tested to meet these OBDII requirements, but in this case, on cold mornings it's so close, that the slightest thing being off can cause it to take a few minutes longer than it should, and the code is tripped.

So to add to what Ratchet said, the problem is the engine is taking longer than it should to warm up. At least where the sensor is reading the temp. In the past, no one would care, and it wouldn't be a problem unless it was a symptom of something worse and if that was the case other symptoms to come up eventually.

But today, since an engine taking too long to warm-up would produce additional emissions, it sets a MIL. Regardless, its a sign that something is wrong, but what's frustrating it could be the result of something just barely being off, which makes it difficult to identify.
I just had this code read at AutoZone. P0128, THERMOSTAT RATIONALITY. 2011 WK2 with V6.

Here is my situation: I have MaxCare warranty from Chrysler and already have an appointment with dealer on May 7 (3 weeks from now) for "no heat coming from the vents on the driver side" and 2 recalls. This is the earliest appointment I could get with a loaner car (I must have a car to get to work)

Is it safe to drive (40-45 miles/day) for 3 weeks? Other options include going to my local mechanic and pay out of pocket (I'd rather not) or try to get an earlier appointment with this or another dealer.
You do know what the number one cause of no heat coming out of the vents? Being low on coolant. If the coolant level gets too low it won't circulate through the heater core and thus you won't get any heat. As well, only a little coolant circulating through the heater core will cause only a very little heat. The Modern HVAC systems under the dash are pretty compact, and the air has to make some tight bends. So when the AC or Heat gets really weak, the tight bends often result in air that's heated or cooled making it to one side of vents. As well, leaks can cause only part of the heater core or evaporator to heat/cool and thus air for one side coming out on one side not being heated or cooled. If this is the case, if one side has no heated air, then the other side might have heated air, but its barely heated at all.

If you have a dual climate system, it might simply be the mixing doors for that side got jammed or failed. If that was the case, then the side with heater/cooled air would have fully heated or cooled air.

Since this "could" be a symptom of being low on coolant, why would you wait 3 weeks to check your coolant level?

Keep in mind, the "proper" way to check your coolant level is when the engine is cold, you remove the pressure cap from the radiator and make sure the coolant comes up within an inch of pressure cap mount.

The reservoir (overflow tank) only shows the coolant level if the cooling system is working properly. The manufacturers recommend checking the level at the reservoirs because of liability, to many idiots burn themselves badly opening hot radiators. If there is a leak in the system, when the engine cools and coolant contracts, it will suck air back in through the leak instead of sucking coolant back in from the reservoir. I have seen plenty of cars where the reservoir indicated the cooling system was full, but the cooling system was almost empty.

So check the coolant system level properly, only when its cold, and fill it if its low, make sure to use the proper recommended coolant. Beware the "All makes, all models" crap on the store shelves, which seems to be the only thing you can find anymore. This stuff is crap and not compatible with other anti-freezes, they've been sued for making the claim, and lost, but still keep making the claim.
 

·
Registered
Joined
·
1,399 Posts
I will check coolant level in radiator tomorrow morning. In overflow tank it is about half an inch above "Add" mark - I figured I will have dealer add some.

I knew about WK2 needing special coolant, thanks.
 
41 - 60 of 70 Posts
Top