I would guess the O 2 sensors are working fine and they told the engine management system the car was rich or lean. The engine management system then used software to compensate but eventually it ran out of capability to compensate for the problem. The real problem is that you have only seen the tip of the iceberg in diagnostic data. You don't even know rich or lean. What you now need to know is which adaptive limit was reached and which way it was compensating.

The answer lies in the current adaptation numbers. Go tell AZ that you want adaptation numbers in enhanced data, not generic BS.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician