Catalyst Monitor (Bad Catalyst)

Fig 1: Bad Output Voltage On Pre/Post - Catalyst Heated Oxygen Sensor

Courtesy of GENERAL MOTORS CORP.

A degraded TWC catalyst, 65 percent hydrocarbon conversion, will show greatly increased activity in the output voltage from the post-catalyst heated oxygen sensor (2). The degraded catalyst post-catalyst HO2S output voltage will therefore appear similar to the typically active output voltage of the pre-catalyst heated oxygen sensor (1).