The black box has a button on it. Press the button for 3 seconds (with the engine off and the ignition in the on position) and release. Count the number of flashes for the fault code. One blink indicates no codes present. Repeat the code retrieval by again depressing the button and count the blinks a second time. If you get the same number then there is only one code. If you get a different number of flashes, then you have more than one code. Keep repeating untill all codes are read. This is indicated by the repeat of the first code received.
Here is a site which shows the fault codes and what they mean:
This code reader only stores codes for faults that are smog emissions related. The black box stores other codes, but you need to make a simple code reader device to pull these codes.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".