Your car drops all the way to zero because it is not in closed loop. If the system is functioning your car is doing this because you are toooo rich. Zero current is the maximun lean correction possible within the lambda system.
I can explain this but that is not to say that your car will run at the required mixture. Most of my life I dealt with cars that in no way could have been made to run as lean as is required to go closed loop at lambda. If you really wish to understand this, give me a 500 word essay explaining that last sentence; most importantly: going closed loop at lambda! I feel that most do not understand that concept.
I can't say I blame people for not understanding it. The concept came out in 1979 (with Volvos - 1980 for MB). I worked around the system for five years before in 1984 we started bringing in grey market cars and had to design feedback control systems to feed three way cats. We were tested on the results. Nothing like failure and money to breed understanding. I worked in the dark for nearly 5 years on these systems.
Bosch Master, ASE Master, L1
33 years MB technician