P0128 Code

Anybodyhome

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I've been getting this lately. I know what it is, but I also am trying to answer a couple questions along the way. And here are my various scenarios I've worked through:

1. ECL comes on, run JSCAN and get P0128 code.
2. Clear code.
3. Cold start JK, ECL immediately comes on. Question #1- how is this code thrown when the engine is cold? Whether or not I clear the code is irrelevant, it will immediately show when I start hot or cold.

1. ECL comes on, run JSCAN and get P0128 code.
2. Top off coolant (maybe 1" below hot line), clear code.
3. 1-2 days later, ECL comes on; (you guessed it- same code).
4. ECL immediately throws code upon starting, hot or cold.

1. When ECL came on, as expected, this morning, I ran JSCAN all the way to work on the back roads- 45mph with a few stoplights along the way. Coolant temp never went higher than 187 and was 73 when I started the engine (air temp this morning was 60).

At this point, although I'm not real concerned that my engine is running cooler than it should, I'll order a new OEM sensor and likely a new thermostat as well. I'll start with the new sensor for a few days and then pop in the new thermostat if that doesn't solve anything.
 
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Anybodyhome

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Is the cooling fans coming on to early? My cooling temps usually stay around 190 to 195.

Not sure when the fan is kicking in, but the temp gauge tends to stall out around the 1/4 mark and isn't up its normal 1/2 mark until a bit down the road- seems to take a little longer and the heat isn't as warm as it normally is, so I'm thinking the t-stat may be sticking. But I'll check the easier small stuff first.
Does the JSCAN heating/ventilation module tell you when the fan kicks in?
 

BLACKJKU

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Could be the T-stat, might as well replace the temp sensor while your doing the t-stat just in case. I have heard of them failing on some of the JK's.

I don't have JSCAN so can't answer that question.

Also according to this your right on track with the T-stat or sensor.

 
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Could be the T-stat, might as well replace the temp sensor while your doing the t-stat just in case. I have heard of them failing on some of the JK's.

I don't have JSCAN so can't answer that question.

Also according to this your right on track with the T-stat or sensor.

I'm ordering both OEM and will replace the temp sensor this weekend, drive it for a week and replace t-stat next weekend if there's no change.
I believe JSCAN will allow me to set the temps at which the fan cuts on, but I'll not change that from the factory settings since the engine is all stock except for the tranny cooler.
 
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Anybodyhome

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Tonite when coming home from work, I cleared the code, which present upon cold start. Started the Rhino and took the interstate home: 9 miles @ 70mph cruise control @ 2200 rpm. Highest coolant temp on JSCAN was 192.

I dunno why it's throwing the code unless it's a bad sensor.
 

BLACKJKU

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Have you checked your outside temperature reading on your dash, seems like I heard something that if it wasn't working or reading correct it can cause a problem.
 
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Anybodyhome

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Have you checked your outside temperature reading on your dash, seems like I heard something that if it wasn't working or reading correct it can cause a problem.
Yes, ambient air temp is right on target. But, now that you mention it, when I cold start with JSCAN showing me readings, the coolant temp is showing 70+ degrees before I even crank the engine even when it's in the 40's or 50's overnite.
I'll match everything again this morning before I had in to work.
 
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I believe I have found the issue: a stuck open thermostat.
The fan does not run early and I checked the temp of the top radiator hose as the engine warmed and it was warming at the same rate the engine temps were reading on JSCAN.
Also, coolant level at cold start was at the hot level in the reservoir. These both lead to the idea coolant is constantly circulating regardless of engine temp
Not a difficult fix, but getting the correct part is a pain- nobody local has the thermostat and housing, so I ordered an OEM fro Partsgeek- about $15 cheaper than Mopar Parts.
 
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Update:
Easily fixed this morning. Replacing thermostat was quick and easy and once finished, I fired up the Rhino and JScan while watching the new T-stat for any leaks, checking the temp gauge as well.
Initial issue was the coolant temp was reading 185-190 tops and I was forever getting the P0128 code. I diagnosed as a stuck open t-stat based on all the info I had.
The upper radiator hose remained cool and when the coolant temp hit 217, I felt the t-stat open and the coolant temp dropped to about 199 and remained consistent, and this while in the driveway.

Here are pics of the t-stat I replaced. You can see the discoloration and surface corrosion, which is why it was probably stuck open.

One other note: I checked 3 different sources for the 10mm bolt torque specs, and one of those sources incorrectly specified 108 foot pounds. It's actually 108 inch pounds. You'll break the bolts if you try to get 108 ft lbs.
Hand tight, no more.

T-stat 1.jpg
T-stat 2.jpg
T-stat 3.jpg
 
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Danarch

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@Anybodyhome have you had any more issues with this? My wife’s Sahara CEL came on today. She arrived home and I pulled the code, was P0128. It is a 2015, with 88K. I have no idea if any of the coolant components (including coolant) have been replaced. We just bought it about month ago, had all the fluids except coolant changed.

1. The DIC showed as high as 226 on the way home. So I assume once up to temp it is running in the normal range.
2. Heater blew hot air (doors and freedom top off, outside temp around 55-60 this morning). She said it seemed to warm up normal.
3. Not sure about when the cooling fan kicks on, I wouldn’t hear it anyways.
4. Overflow is almost to max line, but it is still warm.
5. Will check radiator when it cools down.

Best I can tell since it reaches operating temperature and not overheating it seems like it would be safe to drive till I get it fixed.

1. Did you have an infrared device to get the temp on the upper hose to see of matched the Jscan app or were you estimating temp by feel?
2. Before replacing the t-stat in your post above are saying the temp never exceeded 185-190? Ours is reaching 226 and then drops back down.
 
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BLACKJKU

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When it gets to 226 degrees you should try and pull over and check to see if the cooling fan is on.
 

Danarch

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When it gets to 226 degrees you should try and pull over and check to see if the cooling fan is on.
Thanks, but since the code is that it isn‘t reaching operating temp, wouldn’t I want to see if its coming on too early not at 226. I guess a way to check would be to run the engine with Jscan connected and see what temp the fan comes on. I may give that a try tonight. The cooling fan is coming on as the temp goes back down like it should, there isn‘t an overheating issue.
 
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Anybodyhome

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@Anybodyhome have you had any more issues with this? My wife’s Sahara CEL came on today. She arrived home and I pulled the code, was P0128. It is a 2015, with 88K. I have no idea if any of the coolant components (including coolant) have been replaced. We just bought it about month ago, had all the fluids except coolant changed.

1. The DIC showed as high as 226 on the way home. So I assume once up to temp it is running in the normal range.
2. Heater blew hot air (doors and freedom top off, outside temp around 55-60 this morning). She said it seemed to warm up normal.
3. Not sure about when the cooling fan kicks on, I wouldn’t hear it anyways.
4. Overflow is almost to max line, but it is still warm.
5. Will check radiator when it cools down.

Best I can tell since it reaches operating temperature and not overheating it seems like it would be safe to drive till I get it fixed.

1. Did you have an infrared device to get the temp on the upper hose to see of matched the Jscan app or were you estimating temp by feel?
2. Before replacing the t-stat in your post above are saying the temp never exceeded 185-190? Ours is reaching 226 and then drops back down.
No problems whatsoever.

1 of 2 things is happening when your temp is getting to 226 and dropping- either that's when the t-stat is opening and allowing coolant to flow and the temp drops...
OR
the fan is coming on and cooling the temp

When I was testing to see if my t-stat was stuck open, I started the Jeep cold and wrapped my hand around the upper radiator hose. If the radiator is operating correctly, you should feel no temp increase, nor should you feel any coolant flowing through the hose. When the t-stat opens, you'll feel the flow of coolant and that's when the hose will begin to warm up as the coolant is then cycling thru a hot engine. I had my phone right in front of me with my hand wrapped around the hose and I was watching the engine temp rise on JScan. Once I replaced the t-stat and housing, I duplicated this and, again with my hand wrapped around the upper hose, I could feel the t-stat pop at the correct temp and began feeling the flow of coolant thru the hose.

If your temps are reaching 226, I can guarantee you the t-stat is not stuck open. If the t-stat is stuck open, you feel the flow of coolant immediately and it will get warm within a few minutes. With just close to enough coolant in the system, there's little chance the temp gets beyond 200 because the coolant is remaining in the engine block and is immediately cycling as soon as the engine is started.

My t-stat was stuck open, the temps never went above 190 and I had a CEL light. I used my JScan to catch the code and went to work from there. By the way, I believe the JKU t-stat is set to open around 200-205.
 
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Thanks, but since the code is that it isn‘t reaching operating temp, wouldn’t I want to see if its coming on too early not at 226. I guess a way to check would be to run the engine with Jscan connected and see what temp the fan comes on. I may give that a try tonight. The cooling fan is coming on as the temp goes back down like it should, there isn‘t an overheating issue.

Well, sort of. Your temps should not be getting that high to begin with. Normal operating temps for a 3.6 is between 190-205.
 

Danarch

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Well, sort of. Your temps should not be getting that high to begin with. Normal operating temps for a 3.6 is between 190-205.
Well have to say the entire temp thing on JK’s baffle me. Seems like opinions are all over the place on the right operating temp. I’ve had my JK for about 7 months now, it seems it normal range when the temp is over 80 degrees is anywhere between 212 and 226. When it hits 226 it drops. My wife’s JK only had about a month and this is the first time we’ve checked it reached 226 and then dropped in temp. For mine overflow is correctly filled and the radiator is full to the cap every time i’ve checked it.

I am not saying its right, just that is what is seems to operate at very consistently.
 

Danarch

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Not sure my post was clear. The CEL is for the P0128 code which is for an engine operating below the designed operating temp. Just trying to figure out possible causes or where to start.

I agree is seems odd that that engineer reaches 226 and heater seems to work okay if the t-stat was stuck open. I’ve had that happen on other cars and they never reach operating temp. So I am little confused as to why the code is for low coolant operating temperature. If it was low then why would it reach 226 before cooling down.

I know that our dash tempature on each of our JK’s read low for about 10 minutes before they are accurate. Is there a way to adjust this? BlackJKU mentioned he heard of it causing issues. Not sure how or why or how to fix it. I’ll do some checking after dinner if I have time.
 
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I am not saying its right, just that is what is seems to operate at very consistently.
It is throwing a code, so that tells me something isn't behaving correctly.
If you have JScan, fire it up and do what I suggested- put your hand on the upper hose and feel when the t-stat pops and what JScan says the temp is when it does.
 

Danarch

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Okay so went out did what you said. The temp on Jscan rose steadily over 6 or 7 minutes to 195, By steady I’d say 2 degrees at a time, not a sudden jump. The heat from the upper hose on my hand rose steadily too, around 180 it was getting uncomfortable to keep my Hand on It. I think water was being pumped through it the entire time (my dad would always try and tell me how, just never seemed to get it), It felt like a gas pump pumping only not as strong as that if that is what it is supposed to feel like.

Once up to 210 or so my son revved the engine to about 2000 RPM. The fan came on at 226.

Thanks in advance.
 

Danarch

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So dump / crazy question. The temps in my JK stay around 200 in the morning when its cool. They only hit the 226 in afternoon, or say temp is over 80. I assume my wife’s does the same. The last few days it has been in the 50‘s here in the mornings. I am wondering if her JK temp hovers around 200, but then she is running the heater full blast since no doors and freedom panels are off could that expel enough heat from the system to keep it cooler then 200, thus making the code come up?

I think I have her keep an eye on the temp this morning and see what it does. The 226 reported yesterday was on the drive home so different scenario.