OBD II Readiness Monitors

Non-Continuous Monitors

Catalytic Converter Monitor

The catalytic converter monitor evaluates the ability of the converter to reduce harmful emissions. When the catalyst in the converter is working correctly, oxygen from the exhaust gas is used up in the breakdown of other components (pollutants) of the exhaust gas. Under normal closed loop fuel control operating conditions, all the oxygen entering the converter is used up in the reaction. The PCM measures the level of oxygen entering and leaving the converter and calculates converter efficiency. This is accomplished by comparing the outputs from the upstream and downstream oxygen sensors. If the converter is functioning, there should be little unburned oxygen left in the exhaust as it exits the converter.

During closed loop fuel control, pulses of oxygen from each cylinder’s combustion event hits the upstream oxygen sensor in very rapid succession. The resultant voltage signal produced by the oxygen sensor oscillates with the oxygen pulses. At the outlet of the catalytic converter, the downstream oxygen sensor measures no oxygen and its signal is a constant voltage indicating all the oxygen has been used up. If the downstream oxygen sensor reading is fluctuating from high to low similar to the front sensor, it means the converter is not functioning. The PCM will illuminate the check engine light if the difference in oxygen sensor signals indicates a drop in converter efficiency has happened that may allow emissions to increase to a level that is 1.5 times the federal certification limit. 

In order for the PCM to enable the catalytic converter monitor the check engine light has to be commanded off, continuous monitors and in most cases, the oxygen sensor monitor must have been successfully completed. The PCM can begin the monitor when the engine is operating under normal operating conditions at steady highway speeds. The PCM adds extra fuel into the catalytic converter through the fuel injectors to get the internal temperature up to proper testing temperatures of over 1000 degrees Fahrenheit. The PCM observes and compares the upstream and downstream oxygen sensor signals. 

The PCM will successfully complete the catalytic converter monitor if the efficiency of the converter is calculated to be above 50%. The PCM will set a pending code if the efficiency of the converter is calculated to be below 50%. The PCM will command the check engine light to illuminate and store a DTC if the fault occurs during 2 drive cycles.