Diagnostic Overview
WARNING: This page does not describe the selected car, but rather 10 other vehicles, including the 2010 BMW M6, 2010 BMW M5, 2009 BMW M6, 2009 BMW M5, and 2008 BMW M6. However, it is still accessible from the selected car via links, so may be relevant.
The ECM tests the catalyst system during steady state driving by cycling the fueling LEAN and then RICH for a calibrated number of cycles while monitoring the oxygen storage capacity (OSC). Prior to the Catalyst test the canister purge valve is almost closed. This is to eliminate the influence of canister vapors on the downstream sensor during the test procedure.
The first lean to rich cycles of the test is only used to establish an average voltage value of the downstream sensor voltage. During subsequent cycles the OSC is based on the integrated (accumulated) value of the difference between the average value of the previous lean to rich cycle and the measured instantaneous voltage during the current lean to rich cycle.