Catalyst check
The catalyst check is based on the direct measurement of the catalyst's OSC during the transition from a rich to a lean air-fuel mixture. The required set up is depicted in Fig 1. The air-fuel ratio can be precisely determined with the front oxygen sensor. The rear oxygen sensor delivers information about the OSC of the catalyst. In a two step process oxygen is first completely flushed out of the catalyst with a rich air-fuel mixture. The rear oxygen sensor indicates this with a voltage signal that is greater than a calibrated value. In the second step a lean air-fuel mixture is flushed into the catalyst and the amount of oxygen stored is calculated1 right up to the oxygen overflow point.
1 OSC(t) = air mass flow * [(normalized A/F ratio) -1] * dt
Oxygen overflow is indicated by a drop in voltage (below a calibrated minimum) of the rear oxygen sensor's signal.