i have had my check engine light on for quite a while. at first i had the codes checked out and it said misfire cylinder 2. i replaced spark plugs w/ bosch platinum +2, and soon after it ran very poorly, slow acceleration, slug speed. someone told me that the plugs were garbage so i ordered the ones that are supposed to go in to begin with. still the light was on. i brought it to autozone to read the computer and they said it came up w/ secondary air injection malfunction, fuel pump control out of range, and another i cannot remember. the car has 82,000 miles on it and i'm not sure whether to replace the o2 sensor or the mass airflow sensor. i have to press down hard on the pedal in order for it to downshift and it moves nowhere near as fast as it used to when it was working properly. the odd thing is that when the engine is cold the car definatly gets up to speed a lot faster, almost like it should normally, except the sudden jerk when shifting. any information would be extremly helpful. also, i'm not trying to spend a whole bunch of money at the benz dealership as i am only 19 living on my own and cash is not abundent.