2012 TAHOE HYBRID CODES P0AC4, U186A, U186B

[Tahoeman12]

Good morning forum members,
I've not found much info on my condition but here's the short of it. Back in March I was going about 75mph on the Garden State Parkway when I got the traction control and reduced engine mode dash warning lights/errors when it all just shut down on me in the far left lane no less. I disconnected the battery for about 5 mins and that reset everything and I was able to continue my trip and I hadn't had this happen until last week when it happened everyday and now it's not safe to drive it. When I get the error lights and the truck won't drive while in gear once I disconnect the battery and reconnect it, it always resets everything and I can drive it home. The above codes are what came up last week. I will also say that a new dash error came up saying service power steering pump.

I'm wondering if it's a loose ground somewhere but so far my DIY mechanic hasn't been able to look at the truck yet and I don't want to take it to the dealership. I assume I need to fix the P0AC4 code first and that perhaps might take care of the other two codes? I actually have the truck up for sale and I have an interested buyer but I cannot sell it with these symptoms.

My AI google search assistant is telling me to check the auxiliary transmission fluid pump control module. this can affect the hybrid system performance.

Anyone else experience these combinations of codes?

Thank you in advance

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please. If we can't see it, it does not exist.

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in.

 

[Tahoeman12]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please. If we can't see it, it does not exist.

I cannot personally assist you with your issue, but other members of this Forum much more knowledgeable than me in this area will chime in.

thank you for the welcome!

 

[Doubeleive]

Good morning forum members,
I've not found much info on my condition but here's the short of it. Back in March I was going about 75mph on the Garden State Parkway when I got the traction control and reduced engine mode dash warning lights/errors when it all just shut down on me in the far left lane no less. I disconnected the battery for about 5 mins and that reset everything and I was able to continue my trip and I hadn't had this happen until last week when it happened everyday and now it's not safe to drive it. When I get the error lights and the truck won't drive while in gear once I disconnect the battery and reconnect it, it always resets everything and I can drive it home. The above codes are what came up last week. I will also say that a new dash error came up saying service power steering pump.

I'm wondering if it's a loose ground somewhere but so far my DIY mechanic hasn't been able to look at the truck yet and I don't want to take it to the dealership. I assume I need to fix the P0AC4 code first and that perhaps might take care of the other two codes? I actually have the truck up for sale and I have an interested buyer but I cannot sell it with these symptoms.

My AI google search assistant is telling me to check the auxiliary transmission fluid pump control module. this can affect the hybrid system performance.

Anyone else experience these combinations of codes?

Thank you in advance

a few videos point to the hybrid battery pack cooling fan being a issue for P0ac4
the brake issue could maybe be a side effect, do you have any aftermarket equipment plugged into the obd port?

 

[Tahoeman12]

a few videos point to the hybrid battery pack cooling fan being a issue for P0ac4
the brake issue could maybe be a side effect, do you have any aftermarket equipment plugged into the obd port?

ok, I do not have any aftermarket equipment tied to the OBD. I'll look into the fan idea, thank you.

 

[Tahoeman12]

just an update, my mechanic couldn't diagnose the problem(s) so I'm taking it to the local shop that I trust. I cleaned the throttle body which didn't really do much. I thought about replacing it but do not want to throw parts at it without knowing it's the problem. I'll keep this post updated. Thank you.

 

[j91z28d1]

I'll have to look up your codes, but the biggest thing is find a scanner that can read the snap shot of live data that was saved with the code sets. that's is pretty important with then trucks.


also check here for the conditions needed to set the codes.

 

[Tahoeman12]

I'll have to look up your codes, but the biggest thing is find a scanner that can read the snap shot of live data that was saved with the code sets. that's is pretty important with then trucks.


also check here for the conditions needed to set the codes.

ok, the problem is in order to get home after the truck went into that limp home mode I had to disconnect the battery to get out of the lane of traffic and get home. by then I had zero codes for him to check. he did put in live data mode but everything checked out ok. bizarre.

 

[j91z28d1]

ok, the problem is in order to get home after the truck went into that limp home mode I had to disconnect the battery to get out of the lane of traffic and get home. by then I had zero codes for him to check. he did put in live data mode but everything checked out ok. bizarre.

usually unhooking a battery doesn't erase codes like it did in the obd1 days. you might need a scanner to read codes from all the modules. the cheaper ones only read the ecm. these hybrids have almost 30 modules. all with codes

 

[Tahoeman12]

usually unhooking a battery doesn't erase codes like it did in the obd1 days. you might need a scanner to read codes from all the modules. the cheaper ones only read the ecm. these hybrids have almost 30 modules. all with codes

Ok. You're probably right. I'm dropping off the truck tomorrow at my local shop. I'll post back with an update.

 

[Doubeleive]

It really depends on the "type" of code, some systems are self-checking, such as srs and abs for instance. If there is no active problem there is no code and there is no historical code saved. many other codes will be saved in history even if the battery is disconnected. you just have to look for history codes specifically.
some will stay in history for a limited amount of key-on or engine-run cycles, some will stay for hundreds of cycles.
codes that say permanent will stay until whatever system has successfully passed self testing xx times in a row then they will go away, even if the battery is disconnected.

so if you are getting erratic or intermittent codes, cel, errors messages that seem to disappear, the trick is to scan while the problem is shown or active before you cycle the key

 

[Tahoeman12]

just an update, it's going to the dealership on Wednesday as my local shop doesn't have the proper software tools to clear the main code which is the P0ac4 code, also it being the Hybrid model he didn't want to touch the truck to be honest. Will see what the dealer thinks.

 

[j91z28d1]

just an update, it's going to the dealership on Wednesday as my local shop doesn't have the proper software tools to clear the main code which is the P0ac4 code, also it being the Hybrid model he didn't want to touch the truck to be honest. Will see what the dealer thinks.

I'm sorry to hear that. sadly this is probably the end of your truck. a dealer will most likely just clean your pockets and not fix anything. fingers crossed you have better luck than everyone else.


the P0AC4 is not the main code, it's the most basic code there is. it's basically just the module for the hybrid sending a request to the ecm to turn on the check engine light. because the ecm is the only module that can turn that light on the dash. the other 30 or so in the truck send a request over the can bus network to light the check engine light.


U codes are all communication codes. meaning the network has an issue. there's a high speed and low speed network.

u186a is lost communication with transfercase and u186b is lost communication with the tcm (tranmission control module) its inside the tranny. you have a wiring issue of some kind. there's a gm technical service bulletin about the big plug on the side of the tranny rubbing a exhaust shield and damaging the harness. but my 2012 year, most of tust was fixed during production. same with the wiring to the aux tranny pump. your 2012 wiring was the fix for the older 08-10s. shouldn't really be an issue.

the loss of power steering code can mean a lot of things, I got that once when my old hybrid battery cells was so bad, under regen the voltage would spike above the max allowed and it would open the main battery contacts as a safety to keep from over charging. the down side is it kills the entire time truck, usually just as you come to a stop at a red light. losing your power assets and stuff. it's not very safe, but gm did it. during that one I got the snap shot showing the cell over 20 something volts, when it should be 16ish at most. but it had the power steering code too. that came up because the power steering modual and it's assist stuff all lost voltage in dramatic fashion when the hybrid battery contacts were opened. nothing was actually wrong with it.

that's what's so hard with trouble shooting these old trucks. it's basically a prius copy electrically, but designed by gm with horrible Diagnostic routines, almost zero training for techs. they would basically force one new tech that couldn't say no, to go to a 2 day class about them. the old good techs didn't want the hassle, knowing under flat rate electrical Gremlins pay nothing. on top of that, those few guys thst took those classes back in 07 dont even exist anymore.

if there's any advice you can get them, is monitor of the Hybrid battery cell voltages while driving, to check their condition. that's usually the main issue of the whole system. and gm not wanting to replace every battery pack under warranty at 80k miles, they kept pushing the spec for what's a good enough battery so far out, the truck could run horrible and still not trigger a code requiring warranty repair. once the battery is confirmed good enough, start with harness tsp and go from there t/s what could cause can bus issues. especially if you have thst shock air ride module in the back under the spare tire and the truck lived up north. the can bus terminates in that module and the module gets full of salt and stuff causing network issues.


but that's all a guess workout seeing it. I kinda love my old hybrid yukon. but sadly I know when something happens I can't fix myself. it's probably going to be parted out. the engine is basically an ls2 for the hot rod guys and the tranny is gold for other hybrid owners, since no one sells them or rebuilds tkem anymore. if the tranny is good, it's kinda the life blood of the truck. luckily it's down sized copy of a Allison bus hybrid transmission and usually outlive the trucks with routine maintenance

 

[Tahoeman12]

thank you for your detailed response. Would the fact that I go into limp mode and lose all gears forward and reverse point to the transmission control module as being my problem? I can't afford not to fix this as I have a buyer for this truck and I need the funds. Caught between a rock and hard place.

 

[j91z28d1]

yeah, I would think that's logical to look at the tcm. my guess would be wiring going to it. but hard to say.

if you can find a shop with a good scanner or a old school tech 2. if there's no tcm code saved there's live data from the tcm that can be viewed while driving. if you can make it happen while someone watches the data that might be helpful.

 

[Tahoeman12]

yeah, I would think that's logical to look at the tcm. my guess would be wiring going to it. but hard to say.

if you can find a shop with a good scanner or a old school tech 2. if there's no tcm code saved there's live data from the tcm that can be viewed while driving. if you can make it happen while someone watches the data that might be helpful.

so, the truck has been sitting at the dealership for 10 days and they can't get the truck to duplicate any of my issues. Weird that with all the codes that they pulled up they can't do a little bit of diagnosing but they called and asked me to come pick it up. I asked if they can restore the software or look for an update and he said no. What do I do now?

 

[j91z28d1]

they can absolutely look for updates to the software. they just don't want to. you can yourself if you want using a 150$ dongle and 50$ access to gm's acdelco website for updates. sadly I don't think it will help you thou.

at this point I have no idea unless you find a mechanic that likes hybrids local to you. or want to become one yourself.

you can buy a China tech 2 for 200-300$ and monthly access to aldata or other software based shop manauls/trouble shooting guides. and start. following the trouble trees.


sadly this seems to be why most end up getting parted out.. great design. old tech hybrid battery pack(should have been lithium) not up to the demand of a full size truck/suv and total and complete lack of repair support by the dealerships. I believe that's what made the prius work, and the Tahoe fail.. dealer support just wasn't there.

I hope you figure it out and can share what you find.

 

[Tahoeman12]

they can absolutely look for updates to the software. they just don't want to. you can yourself if you want using a 150$ dongle and 50$ access to gm's acdelco website for updates. sadly I don't think it will help you thou.

at this point I have no idea unless you find a mechanic that likes hybrids local to you. or want to become one yourself.

you can buy a China tech 2 for 200-300$ and monthly access to aldata or other software based shop manauls/trouble shooting guides. and start. following the trouble trees.


sadly this seems to be why most end up getting parted out.. great design. old tech hybrid battery pack(should have been lithium) not up to the demand of a full size truck/suv and total and complete lack of repair support by the dealerships. I believe that's what made the prius work, and the Tahoe fail.. dealer support just wasn't there.

I hope you figure it out and can share what you find.

ugh. Ok, thank you. I'll keep trying to figure this out. I'm super bummed the dealership wasn't much help. He actually told me they don't really work on vehicles over 10 years old either. What a racket.

 

[the 18th letter]

ugh. Ok, thank you. I'll keep trying to figure this out. I'm super bummed the dealership wasn't much help. He actually told me they don't really work on vehicles over 10 years old either. What a racket.

They only work on 2017’s and up? That dealer must have a wonderful clientele. Information would ha e been more useful before you left it with them 10 days. Sounds like they did nothing at all.

 

[Tahoeman12]

They only work on 2017’s and up? That dealer must have a wonderful clientele. Information would ha e been more useful before you left it with them 10 days. Sounds like they did nothing at all.

yep, they took it for a test drive and couldn't duplicate my issues and parked it and told me to come get it.