engine light came on. I had just fill gas tank and added fuel injection cleaner,test read P0420

Question

2000 Ford Contour Sport check engine light came on. I had just fill gas tank and added fuel injection cleaner,test read P0420

Answer 1

P0420 Catalyst effieciency low

This is catalytic degrade,  The downstream 02 sensor which is a catlytic monitor sensor ,when the frequency of the downstream 02 sensor matches that of the front 02 sensor the ECM make caculation of the  catalytic unable to utilizes oxygen. which is a result of a catlaytic degrade,