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The trouble code P0420 appearing on a 2001 Toyota Corolla indicates that the catalytic converter is not functioning as efficiently as required for emission control. Essentially, the catalytic converter is not performing its task at optimal levels. Specifically, there are oxygen (O2) sensors located before and after the catalytic converter on the same side or bank. There is an O2 sensor situated both in front (upstream) and behind (downstream) the catalytic converter. Typically, these two O2 sensors will display varying readings while the vehicle is in operation. The upstream sensor is expected to show fluctuating oxygen levels based on the exhaust quantity and the engine's fuel consumption rate.
The P0420 code on a 2001 Toyota Corolla indicates that the downstream oxygen sensor has detected that the catalytic converter is not working at its full efficiency level.