I recently purchased a 98 Subaru Legacy with a 2.2 L engine manual 5 speed transmission. When I got the car it already ran pretty good, just a few things here and there that needed tidied up. Engine ran great though. I know the guy who previously owned it hadn't change the oil in a while so I took it to my shop at Honda to change the oil and I figured while I was at it I'd change the trans fluid. I followed the amount of fluids I should put in the vehicle recommended by the owners manual. I drove off and got home. When I got to the house I noticed there was some smoke coming out of the tailpipe. I figured maybe I overfilled just a little bit on the engine oil. I checked the dipstick and it was barely over the full line so I didn't think anything of it. About a week went by and I started noticing that when I begin to accelerate at a dead stop and get to about 2500 RPMs it starts to hesitate until I get past 3000 RPMs. Then it seems to take off rather quickly and continue to accelerate.
I already checked the wiring to the right of the bellhousing to make sure I didn't disconnect any of the cables, I noticed that I had accidentally unplugged the O2 sensor at the front of the car. I've since then plugged it back in with no improvement on driving. The rear O2 sensor has been out since I've had the car and it was driving fine with that one being bad. Someone suggested that maybe I put the wrong weight in trans fluid but I made sure that I put the right kind in and that's not the issue. I've already ran a scanner on it and the only codes that pop up are O2 sensors which have already been there since I bought the car. I haven't changed too much to the car besides change fluids so I'm kind of stumped besides maybe putting too much oil. Also I've since then checked the dipstick and it's right where it needs to be.