I agree with you, on my Ford Explorer, there are down stream O2 sensors and up stream O2 sensors.

On my Ford, down stream O2 sensor locates between Cat and Muffler, someone did try to remove their Cats but it caused down stream O2 sensor reading going out of tolerance, and triggered check engine light.

However, on My ford, down stream O2 sensor is used only to monitor Cat performance, it doesn't involve in engine management, so it doesn't hurt the truck itself (but our environment). The up stream (pre-CAT) O2 sensor does involve in engine management.

If MB only has one downstream O2 sensor, it must be involved in engine management, so I think, it can cause air/gas ratio problem.

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.