How many miles are on it? See if the head gasket and the wiring harness has been changed.
Do a search on the "code reader" under Authur Dalton. He is a good resource for getting you over the learning curve on reading the codes and understanding what is going on. Also, I have found it is not anything to really be intimidated by. In about 40k miles of ownership on my E320 mine has come on once when I needed to replace the O2 sensor and once when I had to "rod out" the egr line from the valve to the intake manifold.

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Jim