Post Catalytic Converter Sensor Signal

The post catalyst O2 sensors monitor the efficiency of the catalyst as a requirement of OBD II. This signal also provides feedback of the pre-catalyst sensors efficiency and can cause the ECM to "trim" the ms injection time to correct for slight deviations.

• If the catalyst is operating efficiently, most of the remaining oxygen in the exhaust gas is burned (lack of O2 - "constant lean signal").

  The sensor signal fluctuates slightly in the higher end of the voltage scale.

• If the post sensor shows excessive fluctuations  (which echo the scope pattern of the pre sensor), this indicates that the catalytic converter is not functioning correctly and cannot consume the O2 (fault set).
• If the post sensor fluctuations move out of the normal voltage "window", this indicates that the pre sensor is not performing properly due to slight  deterioration. These systems can also "trim" the ms injection time to compensate for this.

The constantly changing oxygen sensor input to the ECM is needed to correct the ms injection time to ensure that the ideal air/fuel ratio is maintained.