po420 is cat efficiancy below threshold.. 421 is also a cat code for bank 1 i believe in the case that you have 2 cats.. 1 dog and a turtle..
anyway.. how many miles on the car? any missfires? or been a long time since a tuneup?
more then likely its the cat isnt working right 2nd option is the DS o2 sensor 3rd option is to just get a obd1 car.. emissions suck ass