For my M104 (93 300E) when using the make-it-yourself reader, after reading a code, wait for the flashes to stop then hold switch for 8 seconds and release. Then read next code and repeat. You don't need to wait to long after the code finishes flashing or you will have to try it again. After you have gone through all the codes this way and depending on the pin, you should get 1 flash saying that pin is clear. Look around a bit on the forum, but if yours is the 16 pin connector mine has, that way has always worked for me.


To check your wiring harness, cut back a small area of the outer wrapping near the MAF and the one running across your fuel rail. Inside should be nice colorful wires well insulated. If they are old looking, bend the spot you are looking at and make sure they don't crack. The old stuff will possibly fall apart in your fingers, depending on its condition. Mine was bad enough on the ETA that it was mostly corroded copper and dirt inside the outer wrap.