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98 Honda Accord ECL On With P0420 Error Code

I bought the car from a friend and before he sold it to me; we to get the engine check light (ECL) to turn off. We replaced downstream & upstream o2 sensors (ourselves) and the cat (muffler shop, non OEM).

Afterwards, we had no error codes and no ECL. A few months (Winter time) and I’m moving 1000 miles away and the ECL comes on again- it’s the radiator fans dying. It’s between 30-50 degrees out, so I monitoring the temp gauge, but ignoring the problem until we settle in the new town. ECL is intermittent.

We get settled and I go in for state inspection, emissions diagnostics, and I get the radiator issue taken care of. Turns out the aftermarket o2 sensors did not protect the wiring properly and the salt/water from snow corroded the downflow o2 sensors wiring harness connectors. I had the o2 sensors replaced and they cleaned up the corrosion.

A few days pass and the ECL is on again and is constant, I returned a week later for diagnostic. The shop is getting the P0420 code “Catalyst System Efficiency Below Threshold (Bank 1)” I almost don’t believe it, as I said the parts have been replaced less than a year ago.

I have done some research, checked for exhaust leaks. I think I might have the throttle body checked for this: and cleaned. I spoke with the friend I bought the care from and he doesn’t doesn’t remember having this checked out. The car’s VIN and year model are in line with the Service Bulletin.

Any comments, feedback, or direction is very appreciated! Thanks in advance! :smiley: