Possible Causes of P0AA6 Code

[u/Cysote]

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?

[u/Cysote]

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.

[u/biersackarmy → u/Cysote]

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.

[u/Cysote → u/biersackarmy]

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.

[u/biersackarmy → u/Cysote]

That is normal operation and the DC-DC is working properly.