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