engine code 0420 what is this and what is the possible problem

In my Nissan that meant a Cadalytic Converter. Buy some additive and have the codes reset. A lot of time it's just junk built up on the comb. I did this with mine and the light has never come back on. You have to follow the instructions and get premium fuel or it doesn't work so well.

usually bad cat

0420 means "Catalyst system efficiency below threshold (bank 1)' The OBD 2 system measures oxygen content in the exhaust stream. This is the 02 sensor.1 before the cat and 1 after. More than likely the 1 after the cat needs to be replaced .It's cheaper than a cat. When you try to remove the old one heat it with a torch so the threads will let go and ease removal.

not tring to be mean but i've rarely seen a p0420 code fixed by changing an o2 sensor, the code means the reading between the upstream and downstream sensors are mathcing eachother. They normally wont as the downstream sensor os after the cat and should read cleaner (closer to air). A defective sensor just pegs out in one direction and either wont fluctuate or fluctuate slowly. Or will throw the heater circuit code