Catalyst Monitor (Good Catalyst)

Fig 1: Output Voltage On Pre/Post-Catalyst Heated Oxygen Sensor (Good Catalyst)

Courtesy of GENERAL MOTORS CORP.

A good TWC catalyst will show a very active output voltage on the pre-catalyst heated oxygen sensor (1). A good catalyst, 95 percent hydrocarbon conversion, will show a relatively flat output voltage on the post-catalyst heated oxygen sensor (2).