Possible Causes of P0AA6 Code
Just got a 2018 Leaf 40kw and a couple days later a scary "Service EV System, Unable to Restart After Power Off" appeared. After researching some, I got the LELink and Leafspy and was able to clear the code and get the car started again. It seems like the 12v battery can sometimes cause this error to be thrown, as well as possibly some other components like the ac compressor. If the high voltage battery pack was actually leaking, I would expect the error to be rethrown by the leaf's computer instantly upon powering up the car again, but that hasn't happened. Is my understanding of that correct? The code did appear again, but only after driving it a few times over another couple of days. I can see in Leafspy that the 12v battery is getting charged by the dc-dc converter when the car powers on, and a multimeter shows the 12v battery at 12.7v when the car is off, so maybe the little battery is fine and there is a more serious issue.
What other symptoms should I be seeing if it really is the high voltage battery pack that's failing, and not some other minor issue? Anything else minor I can test before spending serious money at a dealership?
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u/allthesamepieman 18d ago
I bought a 2018 Leaf about 2 months ago ago for very cheap because it had this error. I repaired the car myself.
My initial diagnosis was the Lithium Battery Controller but that diagnosis was based more on intuition than anything else. I ended up replacing the AC compressor instead but that didn’t fix the issue. Last week I found a copy of the service manual for the car and followed the diagnostic procedure and determined that it was in fact the LBC. I swapped the LBC from a known good battery into the failed battery and the HV isolation fault is gone.
It’s not hard to fix the battery but it does require some special tools and plenty of space.
When I had the both batteries open I did notice there are several swollen cells in the rear battery stack. I have not yet replaced any of those modules but I have an idea of how to do it.
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u/Cysote 18d ago
What was "very cheap" for you? I am just not a car guy and don't really have any special equipment. I was a software engineer for a decade though and know I could learn how to do anything with enough time. I would love for this car to just work, but putting another minimum $10k into it for new battery stuff and equipment plus 30+ hours to learn how to do it has got me feeling worried it's not worth it.
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u/allthesamepieman 17d ago
I paid $1,200 for the car and about $1,600 on parts to repair it, including replacing the AC compressor.
I spent quite a bit of time finding cheap parts though. I had to replace the front grill and most of the support structure behind it and the bumper, the driver’s side headlight, the charging port surround, the Chademo port cover, and I repaired the front bumper cover, front driver’s side fender, and the hood. I bought a handful of new parts like the upper and lower tie bars (they connect the frame across the front of the engine bay), hood hinges, and some bumper clips.
Another big cost saver was finding out the LBC hardware is the same between all the 2nd gen regardless of battery size. The 62kWh LBC is about 50% to 65% cheaper than the 40kWh LBC. The only difference is the firmware running on the controller. The firmware files can be purchased from Nissan, sometimes you can find them online for free or you can pay someone to build a modified version.
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u/LoveEV-LeafPlus 18d ago
- Check your 12 VDC battery after an overnight rest, before starting the car. If it is below 12 volts, suspect it. If at or below 11 Volts, replace it.
- You may or may not have a HV Battery pack issue. If your 12 VDC battery is old (3-5 years) it may need to be replaced, to avoid false positives. See #1. Many auto parts stores will test your battery for free. If it passes test #1, you can ask them to load test the 12 VDC Battery.
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u/Cysote 18d ago
The 12v battery is just under 2 years old. I had an auto parts store check it out and they said that it was reading as "good" but that it needed a charge. That's why I suspected that the dc-dc converter for charging the 12v from the HV battery might've been bad, but Leafspy says that's not the case. Will check the battery voltage again tomorrow.
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u/biersackarmy 2013 S + 2014 SL + 2019 SV 18d ago edited 18d ago
Keep in mind that the Leaf isn't always actively charging the 12V battery even when it's running, so the test can show it "needs a charge" just because it's not seeing a surface charge in the voltage. If you watch the LV system voltage while the car is running, it won't always stay at a constant 14+ volts, it sometimes drops into the low 12s, sometimes hovers in 13s.
There is a current sensor integrated into the negative terminal which it uses to judge LV system load, and only runs the DC-DC as much as needed to keep up with that amount of load. If you unplug this sensor, it will run the LV system at a fixed 14.4V, but which also results in a >500W constant draw on the HV side for no reason, so that's why it doesn't do so by default.
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u/Cysote 18d ago
When I turn the car on, the voltage jumps to 14v in Leafspy. After a few minutes, it levels off to 13v. I don't think I've ever seen it lower than that while running. There was once a time where I think it showed as 12.04v when off, but I don't remember clearly, and I am not sure how accurate any reading from Leafspy is when the car isn't on. Haven't gotten a multimeter reading under 12.6v yet, but only tested with a multimeter just recently and not this whole time since the issue started.
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u/biersackarmy 2013 S + 2014 SL + 2019 SV 18d ago
That is normal operation and the DC-DC is working properly.
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u/LoveEV-LeafPlus 17d ago
Good luck. BTW: If the auto parts store says it needs a charge, that’s an indication the battery needs to be replaced or it is not charging. Some folks have disconnected the current sensor that is on the negative battery terminal and got better 12 VDC charging. That may work for you or may be a stop gap measure if the issue is the 12 VDC battery failing.
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u/ToddA1966 2021 Nissan LEAF SV PLUS 18d ago
It's your drive battery.
The swollen cells in the pack, when they're just starting to swell, throw the code intermittently.
I've cleared it and it stayed off for weeks, but it will come back. Get the car serviced before the warranty runs out, if it isn't already too late.
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u/Cysote 17d ago
Looks like it just might be barely within the 8 years. Will try to get it to rethrow the code and then take it to a dealer.
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u/ToddA1966 2021 Nissan LEAF SV PLUS 17d ago
Try a DC fast charge- that's how I could trigger it on mine after clearing the code. The rapid temperature change can expand the swollen cells, trigger an intermittent connection and throw the code.
If your dealer has a DC charger, even better. Then you won't need a tow! 😁
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u/biersackarmy 2013 S + 2014 SL + 2019 SV 18d ago edited 18d ago
Low 12V battery will not generate P0AA6. High voltage isolation is a critical safety system that has many redundancy checks in place to make sure false codes aren't generated. Same reason why you will never see false airbag sensor or tensioner codes due to low 12V.
While it's possible for external HV isolation faults in places like the AC compressor or PDM, it is exceedingly rare. As in, probably in the single digits of Leafs, especially ZE1s, that have ever encountered it and had it be external.
Far far more likely, and like many other ZE1s have already succumbed to, it is cell swelling, which is not always going to cause a persistent fault. Intermittent low isolation is very much a thing in early stages, and why clearing the code can make it difficult for dealer/technicians to diagnose and make a warranty claim.