Here, in no particular order, are some of the components/problems I've seen cause CEL's on late model MB's.

1. Mass Air Flow Sensor
2. Oxygen Sensor (you've got four)
3. Purge Valve
4. Charcoal Canister Shut-off Valve
5. Ignition Coil(s)/Wires
6. EGR Valve (lookout for clogs)
7. Air Injection System (a "lazy" O2 sensor could throw this code, be careful)
8. Manifold Pressure Sensor (very rare)
9. Transmission (since there's no Transmission warning light, the CEL is used)

Keep in mind that some of the above mentioned components can fail either electrically, or mechanically. #'s 3, 4, and 6 all have internal moving parts. They may recieve the correct signal, but can still fail to do their job.

The list goes on and on. Have the car checked for fault codes and post the results.