I have a 93 300E and I have not yet been able to find the push button or the led that is mentioned in this thread about checking codes. Maybe I'm blind, any help is appreciated. Thanks.

Open the hood--all the way up (i.e. vertical). Look on the top of the engine on the passenger's side and way back by the firewall.
You'll find small black box on the end of a wire. (It may have fallen down in; you may have to search around).

The black box has clips on two sides and a hinge on the third side. Unfasten the clips and lift the hinged cover--and there be the pushbutton and the LED.

I am having the same issue with a check engine light on, when pushing the led button down i get 1 faint flash. did you ever figure out what was wrong?