Ford 2000 7.3 CE light / Power Loss

I noticed it about 2 weeks ago.. omw to work the check engine light came on and there was a noticeable loss in power. Revs go up I guess to compensate for that? It would go on and off and if I was in the middle of shifting it would rev higher than normal.

Check engine light is now constantly on. Biggest things I notice are loss of power. At idle the strokes sound “crunchier.” Before starting, the “wait to start” light stays on a second or 2 longer.

I just had the truck worked on a couple months ago and they replaced some piece on top of the motor that was fuel related because apparently it was leaking diesel(asking friend for specific part now). Idk if it’s related to this issue but they didn’t change fuel filter.

Resetting check engine light does nothing, it comes right back. I also can’t read the code it says nothing is throwing an error so that doesn’t help..

I plan on cleaning both sets of battery terminals and wires, checking alternator connections, and replacing fuel filter. What else could be causing this lol?

I’ve heard there’s a screen filter in the diesel tank that can bog motor down if it’s clogged? No clue here