I have tested my O2 sensor with better ohmmeter and readings came up to be normal. I also have my duty cycle checked and it was oscillating up and down a bit so I assume lambda system is working. There is one problem: engine runs rich. I get 72 to 82 percent duty cycle reading on either idle or 2500rpms. O2 sensor readings are from .2 to .8 volts and it gives out high HC reading at inspection on idle.
I used to have 2 other MBs where I could adjust mixture with long octagon key through air/fuel meter in the middle of the engine. This car has m119 and air/fuel mixture is adjusted by computer. Does anyone know how to bring my duty cycle to 50% again? What causes the computer to run rich mixture?

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1999 ML430 Silver 135k ml
Smoked LED taillights, Alpine in-dash DVD player, 12" sub, Pioneer components, 2 amps, 4200k HIDs, ML55 headlights, 20" rims, mirror LED blinkers

SOLD. 1996 E320 145k ml. Burgundy

SOLD but miss it everyday 1992 500sel Black
Chime buzzer mod,Alpine CD/MP3 player with stock amp,Euro headlights,S600 grill,20" rims, BEST CAR EVER!

SOLD. 1990 300E White
187k ml,smoked tails,Euro headlights,17rims,Intake,12" sub in first aid kit,alarm/autostart,buzzer mod