The OBD II Catalyst Monitor

A catalytic converter uses oxygen to breakdown harmful pollutants. The catalyst 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.

A 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) while the one 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 waveform on the bottom Bank 1 Sensor 2 is flat with little movement. This is because a good catalytic converter uses the oxygen in the exhaust stream to reduce hydrocarbons and pollutants in the exhaust. It stores oxygen from the exhaust stream, typically when it swings lean.

A 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 is because the oxygen in the exhaust stream is not being used by the converter in the reduction of harmful pollutants.