code P0420
Catalyst system effciency below threshold (bank 1)
How do I fix it?

 

you need to look into getting a new cat for your exhaust... i forgot which bank 1 in.. i think drivers side but it means your cat is on its way out... if you dont think this is the cause or have put a new one in already try reading the voltage on the rear most o2 on bank 1 ( right behind the cat) the sensor may have also gone bad like it did on my 2000 for some reason.