General Overview

The upstream sensor will cause emission increase when its response time increases too much (AIF Loop period or frequency check).

The period of the A/F loop is measured and the number of lean/rich transition are counted. The sum of valid periods is then calculated. The corresponding limit period versus operating point (N, MAF) is acquired.

A failure is declared when the sum of the measured periods exceeds the sum of the corresponding limit.

Fig 1: Oxygen Sensor Monitoring Overview

Courtesy of BMW OF NORTH AMERICA, INC.