I have a circuit that automates reading and erasing codes on '95 and older cars. It interfaces to the diagnostic connector of the car in the same manner as Mercedes' pulse code reader or the home-made LED reader does.
The reader has these functions: read a single code, erase a single code, read all codes, and erase all codes. These work with one system at a time. With read all codes by briefly pressing a button only once the circuit automatically reads all the the stored codes, and when finished it continuously scrolls through them on the LED display.
A picture of the circuit on a "bread board" and the schematic are attached. Parts include 2 integrated circuits, 3 transistors, and 10 resistors. Cost of these is about $5 plus shipping from
www.mouser.com. A circuit board to mount and connect the components, banana connectors, and buttons for the erase and read functions are additional.
If someone is interested in building this send me an email or PM for a parts list and further details. One of the integrated circuits is a microcontroller that needs to have a program loaded. If I am mailed the microcontroller I will do this for free and mail it back, or a programmer can be purchased for about $40 from Mouser.