Anyone seen P1128 Check Engine Light?

[XxsullyxX123]

plugged in my scanner this morning and ambient temp sensor, IAT and Coolant temps were all at ambient. coolant temps rose linearly as expected as I drove off with no issues, no faults. After coming to work, jeep sat for 2 hrs and then i had to go somewhere. ambient and IAT were both elevated (heat soak sitting in the parking lot with a pretty warm engine still) and coolant was still warmish. IAT corrected quickly with fresh air, and ambient as well as i drove off.

No answers, just documenting lol

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[XxsullyxX123]

@mwilk012 food for though...again this morning i cold start and AAT, IAT and ECT temps are all ambient and good. at 13 seconds on cold start, Engine Coolent Temp was in the mid 80s and rising smoothly. That got me thinking. Both of my freeze frames with the wonky IAT and AAT also showed ECT 129 and 160 between the 2 times the CEL set off at 13 seconds. Impossible that the coolant was that warm that fast. also highly unlikely that 3 sensors jam up with incorrect temps intermittently at the same time. Maybe PCM or PCM wiring or fuse could be loose. Fuses are an easy check, but I thought I did that last month. Or is it possible that on start up the PCM is too slow to react and is showing temps from the night before stored at some point as it cooled down?

I wish the freeze frame also had a timestamp from my app, just to prove that the CEL was set at 13 seconds after the cold start. I don't know exactly how the CEL formula works. I had turned the jeep on and off a few times while warm but not hot the night before and left it running under 30 seconds each time and then left it til the AM when i cold started it.

The light 100% didnt illuminate til after my cold start, but i suppose its possible that the conditions for the CEL were met night before while the vehicle was still warm and the light didnt throw immediately? If that is possible, than all 3 of those sensor values couldve been correct and it could still be something else that caused it. Same with the first time the CEL came on. The really high IAT and AAT with a ECT at 160 at 13 seconds at idle not moving could totally be a jeep that was cooling off but still hot that heat soaked the AAT and IAT until they normalized.

Anyway, so far no CEL or pending codes still.

 

[mwilk012]

@mwilk012 food for though...again this morning i cold start and AAT, IAT and ECT temps are all ambient and good. at 13 seconds on cold start, Engine Coolent Temp was in the mid 80s and rising smoothly. That got me thinking. Both of my freeze frames with the wonky IAT and AAT also showed ECT 129 and 160 between the 2 times the CEL set off at 13 seconds. Impossible that the coolant was that warm that fast. also highly unlikely that 3 sensors jam up with incorrect temps intermittently at the same time. Maybe PCM or PCM wiring or fuse could be loose. Fuses are an easy check, but I thought I did that last month. Or is it possible that on start up the PCM is too slow to react and is showing temps from the night before stored at some point as it cooled down?

I wish the freeze frame also had a timestamp from my app, just to prove that the CEL was set at 13 seconds after the cold start. I don't know exactly how the CEL formula works. I had turned the jeep on and off a few times while warm but not hot the night before and left it running under 30 seconds each time and then left it til the AM when i cold started it.

The light 100% didnt illuminate til after my cold start, but i suppose its possible that the conditions for the CEL were met night before while the vehicle was still warm and the light didnt throw immediately? If that is possible, than all 3 of those sensor values couldve been correct and it could still be something else that caused it. Same with the first time the CEL came on. The really high IAT and AAT with a ECT at 160 at 13 seconds at idle not moving could totally be a jeep that was cooling off but still hot that heat soaked the AAT and IAT until they normalized.

Anyway, so far no CEL or pending codes still.

Nah temp readings at the pcm should be near instantaneous. The problem not slow part of the detection circuit is in heating up the Actual sensor resistance element, but that’s on the scale of milliseconds, not what your Jeep is doing.

 

[XxsullyxX123]

1000+ miles since the CEL and it popped back on today. it was cold out and it had been sitting for 2 hrs. Popped up at 18 second after startup. That rules out the intake as it has been stock this entire time. Going to schedule a dealer visit in the near future.

 

[XxsullyxX123]

Heading to the dealer Monday. I will be curious as to the conclusion and keep yall posted. CEL illuminated with ambient, intake temp and ECT all 65-75 degrees on a cool start. makes no sense that it would be trying for closed loop. must be some other test that gets run in there somewhere

 

[XxsullyxX123]

Super frustrating bc the CEL went off again after a few drives cycles early in the week and hasnt come back. praying dealer doesnt clear it and give it back saying they couldnt find anything. theyve been really good with any warranty work ive head done though.


@mwilk012 Interestingly I found out in the JScan app that you can drill down into each module instead of running just a quick scan with a generic small freeze frame. Running the scan under powertrain control gives many more freeze frames with much more data. interestingly, the specific modules freeze frame has some different numbers for Engine Coolant Temp and Ambient vs the generic freeze frame. cant explain that unless the PCM is getting its reading from a different sensor and the generic is just is just taking coolant temp from what the guage says. so ECT showed -40 a few times (which means lost signal to PCM). I do believe there are 2 ECT sensors. ambient also showed -20 and -40 depending on the frame. Both cant be intermittent. maybe a connection somewhere. dealer will figure it out monday, i was just hoping to avoid the hassle!

 

[XxsullyxX123]

FYI, picking it up today. As they went through the diagnostic process they thought it may be the upstream o2 sensor and replaced it, but the light came back on during a test drive (luckily it came back so fast). After exhausting all other options, they believe the PCM hardware is the culprit. One is on order, and I will bring it back when it is in. This makes sense since the PCM would show all sorts of wonky readings for multiple sensors in the freeze frames. Not sure i've read about many bad PCM's in 2020s but it is what it is. This dealership has been great to work with on any issues btw in Bedford NY.

 

[58Willys]

Hopefully a new PCM will end this ordeal for you.

 

[XxsullyxX123]

Hopefully a new PCM will end this ordeal for you.

I’m not optimistic. I feel like the intermittent nature of it seems like the wiring is loose or shorting somewhere. But we’ll see.

 

[mike12086]

I’m not optimistic. I feel like the intermittent nature of it seems like the wiring is loose or shorting somewhere. But we’ll see.

Just wondering if there are any updates or if the PCM solved it? Just had this code come on in my 2019 Cherokee 2.0.

 

[XxsullyxX123]

Just wondering if there are any updates or if the PCM solved it? Just had this code come on in my 2019 Cherokee 2.0.

So knock on wood it hasn’t come back for 5,000 miles. Dealer replaced the upstream o2 sensor but said the light came back after. So they had ordered a new pcm for it. When I got it home for shits and giggles I disconnected the PCM harnesses and checked the wiring pins and put it back. Maybe soemthing was loose bc it never happened again (so far). Intermittent is usually a wiring thing especially without another code. I never went back for PCM when the part came in since I felt no reason to this far.

Any other codes?

 

[mike12086]

Only code. They ordered the o2 and are supposed to be replacing it today. Have the Max Care on it so everything should be covered.

 

[XxsullyxX123]

let me know how it works out

 

[JeepersTodd]

let me know how it works out

Any of you guys making any headway with your Jeeps on this issue? Guess what code mine's setting too. Mine is out of warranty, so I'm trying to sort it out on my own. It's already had the PCM software updated. I've been reading up on the net and even talked with a Jeep tech. Seems like this code is a bit common now for the 2020 Wrangler with the 2.0L. Looks like several folks have thrown PCM's and O2's at theirs to no avail.

Mine has become pretty repeatable setting the code on a cold start after sitting a while. All temp sensors look normal, but I noticed some interesting MAP readings. I'm also fighting a loss of coolant in the primary reservoir, but hopefully fixed that now after tightening up the neck on the pump. Guess that recall only applies to some of the 2.0L's.

Mine will set the code on a cold start after sitting a few hours. I can literally clear the code and restart the engine again and the code won't reset. Multiple times. It only sets the P1128 (only code I'm dealing with) on that first initial start. Will share any more findings if I have any success.

Todd
Auburn, CA

 

[XxsullyxX123]

Any of you guys making any headway with your Jeeps on this issue? Guess what code mine's setting too. Mine is out of warranty, so I'm trying to sort it out on my own. It's already had the PCM software updated. I've been reading up on the net and even talked with a Jeep tech. Seems like this code is a bit common now for the 2020 Wrangler with the 2.0L. Looks like several folks have thrown PCM's and O2's at theirs to no avail.

Mine has become pretty repeatable setting the code on a cold start after sitting a while. All temp sensors look normal, but I noticed some interesting MAP readings. I'm also fighting a loss of coolant in the primary reservoir, but hopefully fixed that now after tightening up the neck on the pump. Guess that recall only applies to some of the 2.0L's.

Mine will set the code on a cold start after sitting a few hours. I can literally clear the code and restart the engine again and the code won't reset. Multiple times. It only sets the P1128 (only code I'm dealing with) on that first initial start. Will share any more findings if I have any success.

Todd
Auburn, CA

mine would only come on every 1000 miles or so after clearing and now its been 6,000 miles and no CEL. that being said, its been colder out. so the jeep expects to be cold on a cold start whereas i was wondering if on a warm day cold start any funky temp readings throw the code. unsure.

As I believe I said before, after they fixed the o2 sensor (and said it came back on for them at the dealer) when i got it home i disconnected both PCM wiring harnesses and check to see if the pins were all seated right and blew on it and popped them back together. was that the fix or coincidence?? i have no idea. but easy and worth a try. Intermittent issues are the hardest to find, but normally mean no component is actually broken (or it would throw a different code along with the P1128) but maybe just a connection is lost for a second in the wiring. its worth a try!

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