All right. So last week my S40 was starting to have issues getting over 3k RPM. One day of driving it was fine, next it won't go over 3k RPM but otherwise runs just fine. A/C died at the same exact time as well.

Knew I'd had a bunch of scheduled maintenance stuff to do (just hit 122k), so I parked her and did the following:

A/C, Accessory, and timing belts. Timing belt tensioner and pulley. Accessory belt tensioner (I broke the torx bit off upon reinstallation). Spark plugs. Oil change.

Car started up, threw a hundred error codes at me, found out the battery was dead.

Bought a new battery. Car starts and idles fine, gave me 5 CEL codes.

I googled around and fixed P0075, P2401, and P2420 by replacing Fuse 35 (it was blown).

Drove the car a few miles, it has no power over 3k. Or I can put my foot to the floor and the engine won't climb over 3k in any gear. It acts normal in neutral.

It lacks acceleration pretty badly. I'm not sure how else to put it.

Got the car back home and I still have these two CEL codes:
P0100, and P0458.

Per another internet suggestion, I checked the fuel pressure and it's fine. It hovers between 54.8 and 55.3 consistently. While driving, while idling, even sitting for an hour, it never moves. Since I was reading numbers anyway, check the MAF and it reads 1.2g/s constantly.

I'm at a bit of a loss as to what to do now. Very welcoming to suggestions!

 

Turns out fuse 35 is blown again. No idea what's causing that to happen.