My Geo once warmed up will not go hardly 5mph

I took it to a mechanic and had them put it on the computer
he said the alterntor was bad and battery, I new the battery was, but not the alterntor. I replaced both and it still did not fix, I was told if both were not working right the computer and sensors would not work properly. When I replaced the battery I left the neg. Off for a little bit to reset the compter. I have no clue, was thinking EGR valve or oxygen sensor. Is there a way to test either one.

Thanks,
Poor Boy with no wheels