Join Date: Jul 2004
Location: Northern Virginia
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Could try resetting the ECM and see if the code comes back. More than 70% of those P0420 do not show up after being reset - "automagically" fixes itself. If the code pops back immediately, then you have a definitely issue causing the CEL. Could be as simple as an exhaust leak or more likely, a bad downstream (post cat) O2 sensor. You'll have to backprobe the sensor to be sure.
It is possible at that mileage that the catalytic converter has gone bad - more so if the car consumes any amount of oil. In that case, start shopping around - depending on the smog laws there - you could be stuck purchasing a cat that is legal for Federal and Cali specs - usually means OEM and expensive..
2002 Toyota Corolla S, 1.8L 1ZZ-FE VVT-i
2003 Toyota Matrix XRS, 1.8L 2ZZ-GE, VVTL-i (RIP)
2009 Toyota Matrix XRS, 2.4L 2AZ-FE VVT-i
2009 Toyota RAV4 Limited, 2.5L 2AR-FE Dual VVT-i