General Overview

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

Fig 1: Oxygen Sensor Monitoring Diagram

Courtesy of BMW OF NORTH AMERICA, INC.

The period of the A/F loop is measured and the number of lean/rich transitions are counted. The sum of valid periods is then calculated.

The corresponding limit period versus operating point (N, MAF) is acquired.

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