1997 Honda Accord SE 4 Cylinder Automatic - 209,667 miles. (I beleive my car is NOT VTEC)

So after 45 minutes of driving yesterday morning, I exited the freeway and the CEL light came on. Car seems to be running fine and MPG's have not obviously dropped. I went to Autozone and read code;

P0420 - Cat System Efficiency Below Threshold - Bank 1

I've done a search on this code and realize it is most likely the CAT but might it be my O2 sensor in the back? It is the original CAT and the rear sensor is also the original. I replaced the front sensor about 2 years ago (non bosch). I had a new muffler and down pipe (from CAT back) installed about 4 months ago. I have recently noticed the car sounds like the jetsons mobile of sorts when idling and driving, but it was a cheap muffler and MUCH better than the growl the old muffler made.

I got under the car today to see if the rear O2 sensor was obviously loose, damaged, etc, and found 1 small air leak right where the cat attaches to the new exhaust pipe. It looks like it had lead or something welded into this leak previously. The leak was obvious by sound and feel at idle, so I'd imagine it really leaks when driving down the freeway which is 90% of my driving.

Now the cat is a rusted mess to be honest, but I'm hoping it's not that. It does not rattle and there is no obvious rattling from inside when I bang on it. (I don't bang hard, but hard enough to hear anything loose inside if there was)

My thought process here is to put some JB WELD on the leak after taking the ECU fuse out, hoping that fixes the problem. I had thought about checking the Rear O2 sensor but I honestly don't have a clue how to use my multimeter to check it. If that's easy to do (pretend you are talking to a 5 year old) then I'll give it a shot.

For the Vets/Experts out there, what would you advise as my next step?

Thanks in advance for the help.

 

I would try to fix the leak you found first. Your fix may be as simple as a bad/worn out gasket that is allowing too much air in and screwing up the rear O2 sensor reading.

 

If the leak is on the cat, it might be worth it to patch. Don't use JBWeld, it can't take the temperature. There's stuff that's made for exhaust, it's more like cement. "Muffler putty?" You can use that together with strips of a beer can with steel hose clamps.

If it's the gasket between the cat & the new pipe, you're better off getting a new gasket.

The O2 sensor actually measures the oxygen DIFFERENCE between inside & outside. So exhaust blowing onto the outside of the sensor will mess up the measurement.

The only thing you can really measure on the sensor is the preheater resistance. But that would throw a different code (P0141).