My car has been sitting for a while, so I wanted to take it out for a ride. I had to jump it to get it started. Everything was fine, ran great as always. I parked it for a couple of hours. When I went to leave, the battery was clearly weak, but it didn't even try to start. The AUX lights all came on, but to turn from the starter.

What I have tried:

I bought a new battery... no luck.

Checked the fuses, they are all fine.

I tried pop starting it... no luck.

I thought it might be the central locking, so i locked and unlocked it a bunch of times... no luck. (I have to use the valet key, as I misplaced mine a couple of weeks ago)

My friend brought over his Peake tool and it read, "Code 42, Firing Curcuit, Belt tensioner, rear middle". I have no idea where that leaves me. I tried looking the code up on unofficialbmw.dotcom but didn't see anything.

Any thoughts? I am thinking it has to be something stupid, as the car was running fine.