Hello everyone, i was driving my 2005 Acura RL on the way home and suddenly a P0420 code comes up. i scanned it with my obd2 scanner and it says catalyst system efficiency below threshold. i remember my mechanic once said that he replaced the 2 catalytic converter's in my car 2-3 years ago so it can't be that. Could it be the 2 oxygen sensors that needs to be replaced? i already checked my air intake and everything looks good including the filter. im not sure if i have any leaks but i doubt it.

 

It is most likely a CAT. If he replaced with aftermarket unit it is most likely already gone bad.
OEM ceramic cat can last 10years+ but most aftermarket metallic core ones 2-3 years. Unless it is high quality one like Gesi or Vibrant core.
Also damaged CAT must be uninstalled asap, it can damage cylinder bores.

 

Somebody correct me if im wrong but it also can be upstream (not downstream)air/fuel (o2)sensor?

 

my obd2 scanner says bank 1, im not sure whether its sensor 1 or 2