OBD II regulations require that the functionality of the Catalyst system be monitored. If the catalyst system deteriorates to a point where vehicle emissions increase by more than 1.5 times the Federal Test Procedure (FTP) standard, the MIL must be activated.
The oxygen content in a catalyst is important for efficient conversion of exhaust gases. When a lean air-fuel (A/F) ratio is present for an extended period of time, the oxygen content in the catalyst can reach a maximum. Conversely, when a rich A/F ratio is present for too long a period, the oxygen content in the catalyst can become totally depleted.
Catalyst operation is dependent on its ability to store and release oxygen needed to complete the emissions-reducing chemical reactions. As a catalyst deteriorates, its ability to store oxygen is reduced. Since the catalyst's ability to store oxygen is somewhat related to proper operation, oxygen storage can be used as an indicator of catalyst performance.