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P0128 code coolant temp low

168K views 69 replies 21 participants last post by  rlefig 
#1 ·
Hi, I have a 2011 3.6 GC and for the past month or two about every once a week or so my Check Engine light came on. It would turn off by itself a day or so later so I never checked it, until now.

This morning I have a P0128 for coolant temp being low. It has not gone away yet and I do not know if this is the same code as before, because I did not check.

Last Friday I installed the Diablo 93 canned tune and I know I read something about the fan staying on. Not sure if that is the issue this time. I am going to check anti freeze tonight.

Any other thoughts, besides replacing thermostat or sensor. And if it is those two what order to go in?

thanks

rob
 
#2 ·
Hi Rob

I'm in the same boat, have the p0128 code maybe 1-2 times a week in the morning only. Going to look into replacing the thermostat first.
 
#3 ·
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.

I see you have a lockpick. I have looked into that. I have the 730N RER radio and would like the ability to enter directions while moving, especially when the passenger is in the car. And also show album art and maybe use a USB for music playback instead of the ipod classic which works good but sometimes loses communication and can be finicky.
 
#59 ·
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.
 
#4 ·
I havent used ac in the morning for a while, just the fan, so not related.

Havent been able to find a pdf instructions on changing the thermostat on the 3.6 yet, please let me know if you do. I'm sure changing it is easy, I'm just worried about bleeding the system of air.

Yes love the lockpick, always travel with wife/kids, so much easier when she can enter directions. We have a sienna that we made some road trips in this year, and what a pita it is not being able to enter when you're moving...
 
#6 · (Edited)
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. ;)
 
#7 ·
So Ratchet, in your opinion replacing t-stat would be the first option over the sensor?
 
#9 ·
rlefig: Yes, P0128 is thermostat rationality. I see this almost on a daily basis on these 3.6 engines. I would definately replace the thermostat first. The repair is very simple.

Note: There is no need to completely drain the engine coolant. You will lose a small amount during the thermostat housing R&R. However, there are a few specific steps that must be performed during the refill process. The steps should be available on WK2jeeps. If not, I can forward them to you.
 
#10 ·
Thanks Ratchet, I will try and find the procedure on wk2jeeps. I have read that we need special coolant so of course get that as well. Any particular t-stat best for the 3.6?
 
#11 ·
Also would the check engine light cause remote start to be disabled. The last couple of days have been cooler and when I attempt to remote start I see "remote start disabled on EVIC". It did work last winter. I will clear the code and try again
 
#12 ·
Yep. If the MIL is on, remote start will be disabled.

Save yourself some headache and get the thermostat/housing from the dealer. Should be around $35.
 
#13 ·
Thanks again Ratchet will get coolant and T-stat from dealer.

Hope you are all not in the flooding rain that has been occurring in the Carolina's
 
#14 ·
No problem.
As for flooding, we got lucky this time. Only a few inches of rain fell here. Back in 99, my entire town flooded. Not fun wading through chest deep water in your living room. It can be fun to ride a jetski through your house though. :)
 
#15 ·
Ratchet if you could forward me the link on t-stat changing from wk2jeeps that would be great. That is a good site never been there before
 
#16 · (Edited)
After you have bolted up new thermostat... cliff note version.

-Remove bleeder screw on top of thermostat housing.
-Proceed to refill radiator with proper coolant mixture (50-50).
-Pause the refill procedure when a steady stream (no air bubbles) comes from the bleeder vent.
-Reinstall and tighten bleeder screw on thermostat housing.
-Continue to top off coolant level in radiator.
-Install radiator cap.
-Top off coolant level in recovery/overflow tank to the left of radiator.
-Start engine (make sure AC is OFF) and let idle untill it reaches operating temperature and electric cooling fan cycles on.
-Shut off engine and let cool for several hours.
-Top off coolant recovery tank if needed.
-Done.
 
#17 ·
Thanks Ratchet,

Got the thermostat from dealer for $44 was going to get anti-freeze from there but it was $24 which seemed like a lot at the time. But now doing more research and seeing that the Zerex G-05 is really the only sub and that was $18 at local store then maybe $24 is not so bad.
 
#18 ·
thermostat replaced, it came with the housing. Upon removing the old t-stat I broke the lower bolt. Luckily there was enough bolt left to PB Blast and vice grip it out. That process took about 2 hours as I wanted to be wicked (ok very) careful. Once I got that out it took about a gallon of coolant/water. I drained it some of it before hand which amounted to a gallon and 6 ounces. will check coolant level again.

Hope this fixes the P0128 code.

Thanks Ratchet and all who offered their time and advice
 
#20 ·
Getting here late but for me the first thing would be to get a scan tool and find out what the temperature really is (with an Android tab or smart phone all you need is Torque Pro ($5) and a Bluetooth dongle ($12)). Then see what the radiator hose feels like (warm/cold/hot) when it comes on.

I'm thinking about changing to a 180F thermostat and am curious at what temperature the low temp error is set.
 
#21 · (Edited)
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)
 
#45 ·
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
 
#22 ·
Tourque is a great app, not sure how much it would have helped since mine was so random coming on and then I believe would stay on for X (maybe 100 or 50) amount of miles and then the check engine light would clear. 2-5 days later it was back on.
 
#23 ·
after almost 2 weeks the P0128 code has not returned (knocking sound on wood) so hopefully it is resolved with the t-stat change
 
#24 ·
Good deal. Usually when that code starts to appear, the thermostat will stick open intermittently untill eventually it will remain stuck open.
 
#26 · (Edited)
Well can eliminate most of the faults if you have a reading (forgot to mention that you should be able to get a coolant temp readout in the EVIC.)

What Torque reads is what the engine thinks the temp is. Personally would be surprised if the fault sets above 82C/180F.

Nice thing about the Torque Pro app is that it can create a log file of parameters you select. Can go home, load on PC, & see when the MIL/CES light comes on and check the surrounding parameters (e.g. coolant temp).

I keep mine running on a dash tablet whenever in the car. Still trying to find the parameter for trans temp though - can dsplay with EVIC so know it is on the bus. Anyone know ?
 
#27 ·
I finally picked up my thermostat and coolant today, was planning on doing the job tomorrow. Thermostat looks good, but did I get the wrong coolant? Not the same part number listed on wk2jeeps. I told them for the same vehicle as the thermostat, and even asked if it was HOAT, and he said yes.
Anybody know if this coolant is okay? It's OVU01524-AB
 
#28 ·
I finally picked up my thermostat and coolant today, was planning on doing the job tomorrow. Thermostat looks good, but did I get the wrong coolant? Not the same part number listed on wk2jeeps. I told them for the same vehicle as the thermostat, and even asked if it was HOAT, and he said yes.

Anybody know if this coolant is okay? It's OVU01524-AB



Is it red color or green or what
 
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