Catalyst Monitoring: Notes

Efficiency of catalyst operation is determined by evaluating oxygen storage capability of catalytic converter using pre and post oxygen sensor signals. A correctly operating catalyst consumes or stores most of oxygen present in exhaust gas. To determine if catalyst is working correctly, signal of post cat oxygen sensor is evaluated over course of several pre-catalytic oxygen sensor oscillations. During evaluation period, signal of post-catalytic sensor must remain within a relatively constant voltage range.

Under normal closed loop operation, changing air/fuel ratio in exhaust gas results in lambda oscillations at pre-catalyst sensor. These oscillations are dampened by oxygen storage activity of catalyst and are reflected at post-catalyst sensor as a fairly stable signal. Depending on how vehicle is being operated at time of evaluation and type of catalyst or coating being used, signal may be in lean or rich voltage range.