2020 JLUR, Warning Lights & Trouble Codes, Looking for Advice

[scarleton]

I’ve got a 2020 JLUR with the 3.6L engine, and I’m dealing with some weird warning lights. The check engine light is on, the yellow (A!) light came on briefly but went away, and now the collision detection light is staying on. I had AutoZone scan it, and the codes that came up were U0121 and U0121-00 for lost communication with the ABS module, C1252-92 pointing to a brake booster pump issue, and U0002-00 for a high-speed CAN Bus problem.

I already checked the brake fluid, and it’s fine. From what I’ve read, this could be an issue with the ABS module or wiring, something going wrong with the brake booster pump, or possibly a CAN Bus communication issue causing multiple systems to act up. I’ve also seen people mention that a weak battery can cause weird electrical problems like this, but I haven’t tested that yet.

Has anyone run into this before? I’d rather not start replacing parts blindly, so I’m hoping to narrow down the problem before tearing into wiring or electronics. Any advice would be appreciated!

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[Phrank]

It's your Jeep's way of telling you that it does not want to go out in public with all those ducks on the dash.

 

[Phrank]

Once you get rid of the ducks, have your battery(ies) properly load tested. If it is (they are) the original batteries, it is (they are) due for replacement regardless of the load test results. If the battery(ies) are newer AND load test fine, the green canbus is the next most likely culprit.

 

[scarleton]

The jeep is the wife's, she likes the ducks Summer of 2023 we replaced both batteries, I am assuming they (both main and aux) are fine, the voltage always seems fine from the dash.

 

[Old Dogger]

C1252-92 is a code related to the chassis system of a vehicle. If ABS has C1252-92 Active or Stored, do not replace the brake system module (ABS). The faults are pointing to vacuum pump system having an issue maintaining vacuum levels. Please check the vacuum pump operation, fusing, wiring, and connector for proper functionality.

The U0121 fault code is a communication error between the Anti-Lock Brake System (ABS) Control Module and other control modules on the vehicle. It is set when communication between the ABS brake system control module and the PCM (Powertrain Control Module) is lost for a specified time. The code may be caused by a faulty ABS Control Module, wiring harness, or related circuits. It is one of the most frequent OBD2 trouble codes.

C1252-92 is a code related to the chassis system of a vehicle. If ABS has C1252-92 Active or Stored, do not replace the brake system module (ABS). The faults are pointing to vacuum pump system having an issue maintaining vacuum levels. Please check the vacuum pump operation, fusing, wiring, and connector for proper functionality.

U0002 Jeep Code Meaning, Causes, Symptoms, & Tech Notes
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[scarleton]

I respect there is a wire harness I need to check, but any idea where it is?

 

[Phrank]

The jeep is the wife's, she likes the ducks Summer of 2023 we replaced both batteries, I am assuming they (both main and aux) are fine, the voltage always seems fine from the dash.

Odds are the batteries are fine at that age, but you won't know for sure until they are properly load tested (properly means they need to be isolated and tested independently of each other). My money is on the canbus - however, being a Rubicon, the sway bar disconnect has a connector that is known to cause similar issues. Look for a wire running under the radiator to the sway bar disconnect - they are known to rub on the radiator and wear through.

 

[scarleton]

Thank you, will check and report back.

 

[alphawolff]

Most likely just a failed vacuum pump

 

[scarleton]

being a Rubicon, the sway bar disconnect has a connector that is known to cause similar issues. Look for a wire running under the radiator to the sway bar disconnect - they are known to rub on the radiator and wear through.

I got under the Jeep and checked that wire — it looked fine. I tried to disconnect it, but it wasn’t obvious how, and I didn’t want to risk breaking anything, so I left it alone.

Then on Wednesday, while my wife was driving home, the power steering went out and a bunch of warning lights came on. We got it to a shop, and when she went to shut it off, the lights went totally bonkers. She did get a video of it, though:

 

[Phrank]

Mine did the exact same thing with the power steering loss. Soon you'll be getting the service shifter message and you won't be able to get it out of park. I still think the canbus is the problem. When it is acting up, remove your glove compartment and wiggle the wires going into the green connector back there and see if anything changes. Even if nothing changes, that canbus can still be the problem. I don't know what kind of shop you took it to, but please do not let them start loading up the parts canon on you. Whatever the culprit is, it's going to be something simple. It's just a matter of finding it.

 

[scarleton]

Mine did the exact same thing with the power steering loss. Soon you'll be getting the service shifter message and you won't be able to get it out of park. I still think the canbus is the problem. When it is acting up, remove your glove compartment and wiggle the wires going into the green connector back there and see if anything changes. Even if nothing changes, that canbus can still be the problem.

Yeah, I keep coming across advice about replacing the two CAN Bus terminals behind the glove box. As a software developer, I know just enough (which isn't much) about the CAN Bus to understand that if something like the power steering electronics fail, it can flood the bus with noise, causing issues like this.

I don't know what kind of shop you took it to...

It's a shop that specializes in hi-lifted off-road vehicles, and they're charging us $185 for an hour of their time, only to tell us they don’t have a clue. I'm not too happy with them, and I don’t understand why my wife keeps wanting to go there. No more!

We live in the Cincinnati, Ohio area. When we had the Jeep in Arizona and suspected transmission issues, I took it to AAMCO Transmissions & Total Car Care. They spent 90 minutes diagnosing it for FREE and found a few options. They suggested a 2-hour, $180 deep dive, and I agreed. The next day (I think they actually spent more like 4+ hours), they said they couldn’t find anything wrong and didn’t charge me a dime. Now that's a place I'll gladly return to!

 

[Phrank]

Just an FYI - I have no idea why this is, but anecdotally there is a much higher chance of failure with the green canbus than the white.

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