1996 Honda Accord Check Engine Light - P0420

This car runs great, gets good gas mileage, doesn’t burn oil at all, and is extremely reliable as an Accord would be… even at 147k miles. However, the CEL coming on was always an issue. But, we’ve been very lucky with clearing it and getting it to pass the emissions test with very good results. We recently discovered that there was an extended emissions warranty. So, last week we had a Honda dealer replaced both 02 sensors under the warranty to fix codes P0171 and P0421. Now code P0420 is causing the CLE to come on. Unfortunately, the Honda dealer says the catalytic converter needs to be replaced with an OME cat, but won't do it under the extended emissions warranty because it's an after market cat from NAPA (this cat only has 2 years and 12k miles on it). I can get an OEM cat from an on-line site for less than half of what the Honda dealer would charge, and can do the work myself. But, is there anything else that can be checked before investing $’s on a new OEM cat? We really don’t want to be screwing around with “clearing the CEL and hoping it passes the emissions test”. We prefer to resolve what’s causing the CEL to come on. After all, it’s designed this was for a reason.