Post Catalytic Converter Sensor Signal

The post-catalyst oxygen sensors monitor the efficiency of the catalyst as a requirement of OBD II. This signal also provides feedback of the pre-catalyst sensor efficiency and can cause the PCM to trim injection time to correct for slight deviations.

On Bosch systems, if catalyst is operating efficiently, most of the remaining oxygen is stored within the catalyst. The signal fluctuates sightly in the higher end of the voltage scale (approximately.6 volt). If the post-sensor shows excessive fluctuation (echoing the scope pattern of the pre-sensor), this indicates that the catalytic converter is not functioning correctly and cannot store the oxygen (fault set). On M5.2-M73 and all M5.2.1 systems, if the post sensor fluctuation moves out of the normal voltage window, this indicates that the pre-sensor is not performing properly due to slight deterioration. These systems trim the ms injection time to compensate for this condition.

On Siemens systems, the function of the post-catalytic converter sensor operates similarly to the Bosch system. These sensors produce a swinging signal but lean-to-rich are at opposite ends of the scale due to the sensor design. When viewing a Siemens post oxygen sensor signal on a scope, the displayed value indicates a lean or excessive oxygen quantity indicating a defective catalytic converter. This difference is due to the catalytic converter construction material and the logic of the Siemens control systems.