Exhaust Catalyst

PCM checks catalyst efficiency by monitoring switching frequency of both front and rear Heated Oxygen Sensors (HO2S). Under normal closed loop fuel conditions, high efficiency catalysts have oxygen storage which makes the switching frequency of the rear HO2S quite slow compared with the frequency of the front HO2S. As catalyst efficiency deteriorates, its ability to store oxygen declines, and the rear HO2S begins to switch more rapidly, approaching the frequency of the front sensor. In general, as catalyst efficiency decreases, the switch ratio increases from a switch ratio of 0 for a low mileage catalyst to a switch ratio of 0.8 or 0.9 for a low-efficiency catalyst. Diagnostic Trouble Code (DTC) P0420 (bank 1) or P0430 (bank 2) will be stored. See appropriate DTC-related testing in appropriate SELF-DIAGNOSTICS article.