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DTC P0420 - catalyst efficiency below threshhold. This basically means that the readings received from the post catalytic convertor O2 sensor are below what they should be. Possible causes:
The vast majority of times it is due to faulty convertor.
Check for exhaust leaks and repair as necessary. If ok, check wiring harness for damage at the downstream O2 sensor.
If the above are ok, replace catalytic convertor.
The catalytic convertor is covered by the Federal Emissions warranty for 8yrs or 80,000 miles, so if you are under 80,000 miles it will be covered by this warranty