chrisehogan wrote:so how does the pcm tell if the emmission is wrong after the cat it only has the post sensor and the fault codes available to the post sensor is limited ie heater fault or bad wiring or slow response.
thanks for the reply
It uses the post cat lambda sensor but it doesn't necessarily compare switch ratio. Newer vehicles often use intrusive testing where the PCM, under certain conditions, forces fuel mixture rich and/or lean and monitors post cat lambda sensor reaction. The issue with using scan data or scope data on the sensors is we really have little idea what the PCM is looking for so how can we make a judgment call? If the PCM is making corrections to extend the life of a degrading cat, can we make that assertion based of the output of the post cat lambda sensor?