W140 LED self diagnostic connector does not blink when pressed
 

I have a 1994 W140, S600 with the self diagnostic connector. With the ignition on and following the instructions for self diagnosis, my LED stays lit when the button is pressed but does not blink at all as it is designed to.

If I hold the button down for at least a minute before removing the key from the igniton, it will turn the check engine light off.

Any ideas?

The led does not blink when you hold it down. It simply lights to tell you it is being held in.
Holding down for 3 sec is an initiation signal to the DM..when you release it, the codes will THEN blink back ...count them.
If No codes come back , you have to wait for a few drive cycles b/c you have cleared it.