P0420 code on 1998 Honda Civic with 1.6L engine

Question:

This car has an intermittent code P0420- catalyst efficiency low coming on and when I checked the downstream oxygen sensor, it was responding properly. I decided to replace the CAT (catalytic converter) buying an aftermarket one from a local muffler shop so I can save money (the CAT from the dealership is very expensive). After road testing, the light came back. The muffler then advised me to replace the downstream oxygen sensor but it did not fix the problem. What should I do?

ATS Adviser:

There is high rate of failure when using aftermarket CAT and most garages recommend using only OEM CAT from Honda especially on 1996 models and up. If you put an oem CAT on this, the code will stop coming back provided the oxygen sensor wires are not shorting out. Also, when testing the oxygen sensors for this engine, the upstream sensor is used for engine fuel management whereas the downstream sensor is used to monitor CAT efficiency.