I have a question regarding my check engine light. My light has been on for a while and yesterday (10/6/09) changed out my spark plugs (H9DCO, Bosch) from the MB dealer and wires as well as having a new O2 sensor put on today. I have searched this forum and 2 other forums about how I can get the light off and check the error codes. I know about disconnecting the battery and reconnecting it, which works for a little while, then the light is back on. I read that you can press a button x amount of seconds under the hood and it will flash x number of times to give you a reading. I have found the button right in front of the battery, but I am unsure of when I need to press it. Should the car be running? should it be off? I have pressed it, but when I do the red light will stay lit for as long as I am holding down the button. So any help on trying to get a code read or fix my check engine light would be greatly appreciated. Thanks.