1998 Accord 2.3L, MIL on, Code found
 

I have a code P0420 which says "Catalyst System Efficiency Below Threshold (Bank 1).

Is it safe to assume I have to replace the catalytic converter?

Got any exhaust leaks? If so fix em.

The rear oxygen sensor measures whether your cat is working. But the O2 sensors actually measure oxygen DIFFERENCE inside vs. outside of the sensor. So an exhaust leak blowing onto the outside of the sensor will throw off the measurement. Same for oil or crud dripping onto the sensor.

No promises, cause it just might be a bad catalyst. But you'll want to rule out the easy stuff first.

Just to follow up on this, I replaced the catalytic converter, and decided to use the OEM/Honda. It works fine now. I chose the Honda because the part that was in the car was a 3rd party and it pretty much failed in 3 years, so hopefully, this will cure the problem permanently.

Thanks for following up. The price can be painful, but OEM converters seem to last a good long time.

I have the same car as the OP, with the same code that keeps coming back after erasing it. Car has 219 k original miles, original engine, cat converer & O2 sensors. I still have to check for any leak, as recommended here. Tkrotchko, did you replace the O2 sensors as well?

I replaced the sensor downstream from the catalytic converter, yes. As long as it was apart, the cost was minor compared with the rest of the job.