Originally Posted by tkfx
Anyone else know?
The upstream sensor, or "feedback", is used for fuel trim, the downstream a catalyst monitor. When graphed, under ideal conditions, the fore sensor will register rapid switching, while the aft sensor a series of shallow, regular peaks and troughs that indicate a lit, functional, healthy catalyst (and therefore a healthy engine management system). They are both interdependent in operation of the malfunction indicator, and compliance of IM240.