[Roostre]

2017 with 88k miles. Zero issues before this. Battery was nearly full and I was in EV mode. Temperature outside about 58F.

I was at a stop light- accelerated from a stop with railroad tracks about 75 feet away. As soon as I hit the RR tracks my dash went crazy and the dreaded "Shift to Park" message displayed. A glance in my rear view had me looking at getting rear ended by a big truck approaching quickly and I could not get to the right due to heavy traffic- luckily I was able to make a hard left turn onto a side street with momentum and get out of danger zone.

The car was acting strangely - it would not shut off. Any time the shifter was moved it said "Shift to Park" and a check engine light was on. I finally got out of the car and used the remote buttons a few times- then got back in and it would shut off, but then it repeated all this on startup. So I got it to shut off again and went for a walk to get the remote away from the car; now it started and would let me drive!

Made it about 3 blocks and it dies again - same scenario.

Starts again and I gingerly drive it about 3 miles home.

Torque App and OBD reader says P0C04 - Powertrain, P0AC4 - Powertrain. Nothing else.
I checked the 12V battery connections and voltage. 13.23V with the car off. Same voltage at each of the connectors that come off the side of the battery.

Dealer can't get me in to see a Volt Tech until May 11th. I'm in a work situation where I'm on call 24/7 for the next few weeks.

Do I dare keep driving it?? Are there some easy things to check that I'm missing?

 

[supchrgamx]

loose ground? maybe

 

[hellsop]

P0AC4

On Gen 1, that's just a "Requested MIL illumination code". There's SOMETHING not being shown to you via those means.

 

[WhatAShocker]

2017 with 88k miles. Zero issues before this. Battery was nearly full and I was in EV mode. Temperature outside about 58F.

I was at a stop light- accelerated from a stop with railroad tracks about 75 feet away. As soon as I hit the RR tracks my dash went crazy and the dreaded "Shift to Park" message displayed. A glance in my rear view had me looking at getting rear ended by a big truck approaching quickly and I could not get to the right due to heavy traffic- luckily I was able to make a hard left turn onto a side street with momentum and get out of danger zone.

The car was acting strangely - it would not shut off. Any time the shifter was moved it said "Shift to Park" and a check engine light was on. I finally got out of the car and used the remote buttons a few times- then got back in and it would shut off, but then it repeated all this on startup. So I got it to shut off again and went for a walk to get the remote away from the car; now it started and would let me drive!

Made it about 3 blocks and it dies again - same scenario.

Starts again and I gingerly drive it about 3 miles home.

Torque App and OBD reader says P0C04 - Powertrain, P0AC4 - Powertrain. Nothing else.
I checked the 12V battery connections and voltage. 13.23V with the car off. Same voltage at each of the connectors that come off the side of the battery.

Dealer can't get me in to see a Volt Tech until May 11th. I'm in a work situation where I'm on call 24/7 for the next few weeks.

Do I dare keep driving it?? Are there some easy things to check that I'm missing?

Do you have OnStar? If so, call them and ask for a remote diagnosis. They will email you all the codes and also list how severe the issue is and how long they recommend you drive with the issue too. It's actually quite an incredible service I have never seen anywhere. It will definitely tell you whether you can drive with confidence or be cautious. Torque Pro is not really that useful for digging into the sub codes like OnStar does.

Sent from my Pixel 4 XL using Tapatalk

 

[obermd]

P0C04 - Drive Motor "B" Current High - the control module for MGB is reporting too much current.

P0AC4 - Hybrid Powertrain Control Module requested MIL (Check Engine) Illumination - this is a secondary code and will clear when the primary P0C04 is corrected.

 

[Roostre]

Do you have OnStar? If so, call them and ask for a remote diagnosis. They will email you all the codes and also list how severe the issue is and how long they recommend you drive with the issue too. It's actually quite an incredible service I have never seen anywhere. It will definitely tell you whether you can drive with confidence or be cautious. Torque Pro is not really that useful for digging into the sub codes like OnStar does.

Sent from my Pixel 4 XL using Tapatalk

So I decided to try this and wanted to "push the button" during the issue. Went for a drive and made a few stops running errands very local. Like 15 miles and 3 stops. It never recurred and the check engine light cleared on its own. Car feels normal. If/when it happens again I will see what On-Star diagnostics says. (I have the cheapest plan, so I hope it is part of the package).

 

[Roostre]

P0C04 - Drive Motor "B" Current High - the control module for MGB is reporting too much current.

P0AC4 - Hybrid Powertrain Control Module requested MIL (Check Engine) Illumination - this is a secondary code and will clear when the primary P0C04 is corrected.

I heard a louder than normal Kuh-Clunk as I crossed the tracks.

Is it possible that the tires spun on the slippery metal rails and then caught traction on the asphalt and caused a current spike in MGB??

As I wrote above - the CEL has cleared after a short trip of errands.

 

[269983]

So I decided to try this and wanted to "push the button" during the issue. Went for a drive and made a few stops running errands very local. Like 15 miles and 3 stops. It never recurred and the check engine light cleared on its own. Car feels normal. If/when it happens again I will see what On-Star diagnostics says. (I have the cheapest plan, so I hope it is part of the package).

If it's under warranty take it in and have it diagnosed. Typically nothing good results when the CEL is illuminated and then is extinguished without a diagnosis as to why it was illuminated.

 

[Roostre]

If it's under warranty take it in and have it diagnosed. Typically nothing good results when the CEL is illuminated and then is extinguished without a diagnosis as to why it was illuminated.

I'll be keeping my appointment in 10 days for sure.

Hate that I can't trust it like I used to and for a while I'll be riding cautiously in the right lane looking for a place to bail safely.

 

[TYmrLutz]

Is it possible that the tires spun on the slippery metal rails and then caught traction on the asphalt and caused a current spike in MGB??

This sounds plausible, especially if acceleration rate was high. I've had this occur but shows up as a low traction message with brake booster activation. Perhaps conditions were severe enough in your case that the main contactor opened briefly and caused the shutdown. You get a star for learning to drive more defensively.

 

[WhatAShocker]

I'll be keeping my appointment in 10 days for sure.

Hate that I can't trust it like I used to and for a while I'll be riding cautiously in the right lane looking for a place to bail safely.

If it was me, because I am that kind of guy, I would record a video and attempt to replicate the conditions of driving over the tracks as closely as possible. If you can trigger it to happen again, such evidence would be crucial to a) have a more rapid diagnosis by the dealer, b) put your mind at ease regarding how safe your car is to drive (may have been a chance event, 3) avoid the dealer telling you, your car was fine and they were unable to replicate the condition.

Give it some thought, though I do acknowledge your need to be available 24/7 however. I am siding with the high speed overspin condition, or car thinking it was momentarily air born, it could theoretically cause it and the car panicked and perhaps over reacted. Again, all theories. Hahaha.

Sent from my Pixel 4 XL using Tapatalk

 

[obermd]

Knowing how rough most railroad crossings are my suspicion is you have a loose power line somewhere that is causing this. It could even be an internal short in the sensor.

 

[bobroth4433]

The worst part of your problem is that it is intermittent. Hardest problems to diagnose. I agree with Shocker to try to replicate the problem while recording it. Also, even though your AGM battery reads good, might be a good time to replace it.

 

[Roostre]

Sitting in the dealership service waiting area right now. Service rep just told me that it’s “a loss of communication error and we can fix it with a reprogram. But it’s not covered under the Voltec warranty. It’s $189.”

I’m not sure how a reprogram could fix a loss of communication unless it is a known issue. If it is a known issue then why would it not be covered under the 100k mile warranty???

 

[TYmrLutz]

I'm sticking with the main contactor theory. I suspect the "reprogram" may be increasing the buffer to better accommodate a momentary power interuption. If a covered Voltec component fails down the line that can attributed to this communication error, maybe they'll compensate you for this repair.

 

[WhatAShocker]

Sitting in the dealership service waiting area right now. Service rep just told me that it’s “a loss of communication error and we can fix it with a reprogram. But it’s not covered under the Voltec warranty. It’s $189.”

I’m not sure how a reprogram could fix a loss of communication unless it is a known issue. If it is a known issue then why would it not be covered under the 100k mile warranty???

Call GM after and insist on a refund. They will do it. But I would ask the dealer why a software fix isn't covered.

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[Roostre]

So... I asked him a ton of questions about what the root cause could be and really wasn't answered. SO I told him I'd like to take it for a second opinion.

At this point he said he would ask the service manager if he would do it as "customer courtesy". I fully expected he would come back with an answer, but 5-7 minutes later I got a text that my car was ready.

He handed me the keys and said "I got him to do it for you on the house". And gave me an invoice. This dealership has always been straight shooters with me and I've recommended them to friends so I'm going to give them the benefit of the doubt here. Something about the whole interaction seemed off, but maybe it was just this rep.

 

[Roostre]

 

[WhatAShocker]

View attachment 172338

Hey that's a win in my books. Remember that these guys deal with so many issues in a day, some legit and some are plain ridiculous people. If you have had a great experience before, then I would say they are a trustworthy dealer. They want repeat business. Some dealers are awful and should be shut down. I am so glad you seem to have a reputable one and had a good solution. Keep us posted if it happens again or how the car drives. Kudos to you and the dealer. Volts are awesome cars and I hope you enjoy yours for many years to come.

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[Roostre]

Just a follow up. 3 months later and now at 94k miles. The car has been flawless and I’ve not seen any repeat of the issue.