check engine light

Its an exhust system problem ither a o2sencer going bad or a catalyst converter clogging up

catalyst system efficiency has degraded below a certain theshold.this is the act.dicription of the code

Have you checked the catalytic converter to see if it seems plugged or if it is getting red hot when the engine is running? The code, PO420, is a generic code that indicates a bad converter, but that isn't always the case. If the down stream O2 sensor (after normal operation temp is reached) has excessive electrical switching activity, it will set a PO420. What you need to check is the down stream O2 sensor. After the vehicle is warmed up (some city and highway driving) the sensor should level and produce around 0.45 volts. If it has fluxuations that reach above or below 0.35 and 0.55 the sensor is most likely the problem and a cheaper fix.

Let me know if you have other questions.

Joe