OBD II Catalytic Converter Monitor

A catalytic converter uses oxygen to break down harmful pollutants. The catalytic converter monitor uses a heated oxygen sensor downstream from the catalytic converter to monitor its efficiency. Vehicle emissions exceeding 1.5 times the applicable tailpipe emission standards result in a fault code and illumination of the check engine light.

Good Catalyst Monitor Waveform

The top waveform above, Bank 1 Sensor 1, is the oxygen sensor upstream to the catalytic converter. Its waveform has frequent repetitive peaks and valleys (voltage). The waveform on the bottom is flat with minimum movement. These fluctuations are caused by the oxygen from each cylinder's combustion event flowing by the pre-catalytic oxygen sensor.

The bottom Bank 1 Sensor 2 waveform is flat with little movement. A good catalytic converter uses the oxygen in the exhaust stream to reduce hydrocarbons and pollutants in the exhaust. It also stores oxygen from the exhaust stream when it swings lean.

Bad Catalyst Monitor Waveform

The image above displays the waveform of a bad catalytic converter. The post-catalytic converter oxygen sensor, Bank 1 Sensor 2, displays similar voltage peaks and valleys as the pre-catalytic oxygen sensor. This display indicates this catalytic converter is not removing pollutants.