What is the oxygen sensor fault code?
A. One of the more common causes of the check engine light is a failing or failed oxygen sensor (also referred to as a lambda probe). The 1995 M3s had only one oxygen sensor (located before the catalytic converter) due to OBD I standards and the 1996-1999 M3s had four oxygen sensors (two before and two located after the catalytic converter) due to more stringent OBD II standards. In short, the oxygen sensor measures the air/fuel mixture and transmits the signal to the DME which attempts to achieve the stoichiometric air:fuel ratio of 14.7:1 (or a Lambda of one). Running the vehicle with a failed oxygen sensor is not overly harmful, but efficiency is reduced because the vehicle will be running excessively rich; a default setting of the DME to reduce any probability of damage when no signal is present.
