Usually my first step is to push down on the airflow plate with my finger and see if the system responds to the richer mixture.
I then disconnect the O2 sensor and verify the ability to give a 50% fault code. If it does then I would try readjusting to the proper point.
One must understand that there is no significant difference inmixture between closed loop at 90% and closed loop at 50%. If the system reached the end of compensation somewhere in the high 90 percentile, immediately it would only be slightly different mixture than at closed loop 90%.
Mixture is held to a very tight tolerance when in closed loop and the car can run perfectly significantly above or below what is achieved in closed loop.
Bosch Master, ASE Master, L1
33 years MB technician