Docaram:

The code reader is really not that hard to build. Just buy the parts listed and lay them out on a table like in the schematic. I did it and I don't have any electronics experience. If you can solder you can do it. Otherwise you will just be guessing.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles