2007~2015 Weird electrical problem

I have a 2010 CX-9 with about 135,000 miles. Last year we had the transfer case issue, and instead of replacing the transmission and differential, we just had our mechanic grind down the splines so that it's now a FWD vehicle. I don't think that has anything to do with this problem, but maybe it's useful context.

Anyway, last week, upon starting up the car after a bit of driving, all of the lights on the dashboard lit up. I drove it home (about a mile), tried to start it, and it wouldn't turn over. Thought it was the battery, charged it up, same thing, all lights come on, turns over, but turn it off and it won't start again. Thought it was maybe the alternator.

I waited until the next day, looked at the battery charge and it was fully charged. Started it up, no dashboard lights, alternator check says it's fine.

I took it to the mechanic for an oil change, he said it was throwing a zillion codes but didn't know what the actual problem was. He cleaned the battery terminals, connected them to each other for a while, and then sent us home. Car seemed fine.

Then yesterday, same thing: drove for about 20 miles, then all lights come on and electrical systems go crazy. Car still drives and shifts fine, and we made it home (about 10 miles), but then same thing, won't turn over. Let it sit for a few hours, and it starts up fine, no lights.

Any ideas what this could be? It seems to be somehow associated with the car getting hot, it doesn't affect the drive system of the car. I don't think the car itself is overheating, there's no indication that it is. Are there any electrical systems that are affected by heat? This is a really weird one to me. Any suggestions appreciated. Not sure what to ask the mechanic to check out and don't want to get into a very expensive fishing expedition.
it certainly has nothing to do with the transfer case but seems like an ecu or something is going wrong. I am afraid that is not going to be an easy one to fix.
it certainly has nothing to do with the transfer case but seems like an ecu or something is going wrong. I am afraid that is not going to be an easy one to fix.
Thanks for the reply! My mechanic thought it might be a ... well, he threw a couple of three-letter acronyms at me fast and furious, ECU might have been one of them. Maybe PCM as well, not sure if that's a thing.

Looks like an ECU runs $220-400 on eBay, is it hard to get at?

And my bigger concern, will the shop have a way of telling which unit is bad from the codes, or are we going to have to throw parts at it until we find the one that's actually failing?
I really am clueless about all of this, this is the first time i see something like that on these cars. Best would be to find an other CX9 and swap over some of the electronic until you find the defective one but that is not an easy task.
2019 CX-9 GT AWD in Soul Red
Have your mechanic print out the codes that he read, and if he can't, go to AutoZone and have them scan it and they will give you a print out.

Swapping the ECU should be easy, accessible from the engine bay. However, make sure that you are buying the correct PCM part number as it has different versions (AWD/FWD, Cali/non-Cali, etc). Best to pull out your current one and look at the PN.


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