So you have a system that responds! The system is saying: I see lean go rich. It is never getting there OR the sensor is lying.
I would gently apply pressure to the airflap and increase mixture. If the car instantly runs poorer as the flap is pushed you are obviously not lean. I would begin to distrust the sensor.
Generally speaking if you aren't stuck on a duty-cycle fault code, a high reading will mean a lean mixture and adjustment is what is necessary. If the thing doesn't respond then I would suspect a grounded O2 sensor.
Bosch Master, ASE Master, L1
33 years MB technician