somewhere in these forums, a very clever MB mechanic explains the BITE diagnostic system, the steps to find the "true" code. the blinks from the built in button only lead to another set of pins which blink with reader but that also is only a general area of fault but closer to true fault and fix. Keep searching the forums and build the code reader, I did and it works great. Good luck, FYI if car runs fine some people disconnect the CEL.