Your Mercedes S420's drive cycle requires it begin at a cold start (coolant temperature below 122 degrees F). If the ECT (engine coolant temperature) sensor is not functioning properly it will not allow a drive cycle to begin.
Also, this OBDII drive cycle requires battery voltage above 11.5 v. Should either system be defective, the Drive Cycle will not run.
Your 1996 Mercedes S420 may have several more specific to the manufacture. Our recommendation would be to have a local MBZ dealer or certified repair shop diagnose your vehicle's failure.
If you feel your Mercedes does not have any operational faults and would like to follow a MBZ specific drive cycle read the attached PDF below.
Mercedes Benz Drive Cycle Readiness Monitors
