A couple of weeks ago a 2002 Toyota Corolla came into the shop with the P0420 code. If you have been working on cars for any period of time, I’m sure your familiar with the P0420 Catalytic Converter Efficiency Below Threshold code.
This seems like it should be a very straight forward code. Most tech’s will skip the diagnostic process all together, and just throw a catalytic converter in it. This is an extremely expensive guess!
The first step I always take is to confirm that the catalytic converter is actually bad. You can usually accomplish this on a quick test drive while monitoring some live data on your scanner.