Z4 & M3
WARNING: This page does not describe the selected car, but rather 37 other vehicles, including the 2005 BMW 760i, 2004 BMW 760Li, 2003 BMW Z8, 2003 BMW Z4, and 2003 BMW X5. However, it is still accessible from the selected car via links, so may be relevant.
Vehicles use Bosch LSH 25 oxygen sensors, The pre-cat oxygen sensors measure residual oxygen content of exhaust gas. The sensors produce a low voltage (0-1000 mV) proportional to the oxygen content that allows the ECM to monitor the air/fuel ratio.