Measuring Oxygen Storage Capacity

A good catalyst will show a relatively flat output of the post-catalyst heated oxygen sensor. A poor catalyst will show substantial peaks and valleys in the sensor's output voltage, indicating that the catalyst is having little affect on the exhaust and both oxygen sensors are sampling similar gases. By monitoring the post-catalyst oxygen sensor voltage fluctuations during 50 pre-catalyst oxygen sensor voltage cycles, it is possible to determine the oxygen storage/release capacity of the catalyst. From this we can determine the overall condition of the catalyst. High oxygen storage capacity indicates a good catalyst, low storage capacity means the catalyst is failing.

If the diagnostic fails on two consecutive driving cycles the MIL will be illuminated and a DTC set.

Like many OBD II diagnostic procedures, it is impossible for you to "manually" evaluate the condition of the catalyst by monitoring the sensor outputs with a scope or scan tool. Only the OBD II diagnostic software built into the ECM has access to all the information needed to make this decision. In this respect OBD II is very much like our SRS (airbag) systems where the system itself must be the final judge of whether or not a problem exists.