Car Won't Turn Back On/Turns on By itself

Hello,

I have been following some previous threads and have some similarities but cannot find my exact issue.

Previously, when I would turn the car off, sometimes it would turn back on right after shutdown. Recently, after shutdown, it will not turn back on at all and require a jumpstart. When I shut it down, not even the anti theft warning light stays on and the car is completely dead. I have troubleshot the following: 12V Battery tests good, no visible corrosion or loose connections (Connector was previously loose, but tightened and seemed to fix for the interim). Inspected C2 fuse, no visible defect. After putting it back in, then the car worked fine for a couple days. Replaced the fuse anyway just as a precaution and the car turned on fine at O'reillys but when I got back home, it would not turn back on. Code scanner picked up a few codes, most relevant being C1241 for brakes? - Low or high power supply - recommends inspecting/clean battery terminals and perform battery test. B1421/B1442 (Air Conditioning solar sensor/air inlet damper) P3102 - Transmission Control ECU, C2122/C2123 - Tire pressure monitor transmitter ID not received.

Used scanner for Hybrid/EV battery Test(1-14) - All cells measured 15.4 except cell 4 measured 15.3 but graph shows green regardless.

Interesting anomalies: When I took a multimeter to the 12v, it sparked slightly and the car turned on (accessory mode). Similarly I disconnected the 12V negative ground for 1 hour and when I screwed it back in, it sparked and the car turned on again. For clarity, the spark is similar to if you were to jump it. Also when the car was completely dead and I went to manually lock the doors in the back by moving the lock button, it turned on as well. At one point when trying to jump it previously with a jumper pack, the car was saying something about parking on a level surface and engaging the parking brake. I believe this was due to the jumper pack being low on battery as I had done it a few times before and when my friend came with his, he jumped the car from the 12v itself, not the terminals near the fuse box and it worked fine. This was random and again I think the jumper pack was just not putting out enough power.

My friend recommended swapping out the main fuse/fusible link and I will be heading to the store in the morning to see if that helps. I had seen a video where the gentleman does this but talks about how the check engine light would turn on when trying to turn on the car. Again, I have no lights at all but once the car turns on it is completely fine.

I had seen a different thread talking about the wire harness running from the 12v being bad, but before I ripped the whole car apart I was open to other suggestions as this seems slightly extreme, but I am running out of ideas.

Also, the driver's side headlight flickers. This may be completely unrelated and I bought a replacement headlight today, but when I went out to the car, I noticed it was completely dead again and didn't bother replacing the bulb as I was focused on the electrical issue of the car not starting.

Power button does not seem to stuck and the key fob battery was changed within the last 3 months. Weather is mild, but not cold enough that it should be an issue.

My guess is whatever should be triggering to keep minimal power after shutdown must be failing, but I am not sure. Will be swapping the main fuse tomorrow, but open to suggestions as well.

Thank you for your time.

- A guy who just wants to drive.