2000 model with 180,000 mi.. Throwing a code PO420-Cat efficiency below threshold. This Cat is original. The car was misfiring. I changed the wires and inspected and re-gaped the plugs. The plugs were very clean and dry. The car is still misfiring. Is the misfire causing the Cat code or is a bad Cat causing the misfire?

 

I think your misfire is causing your cat to throw a code. I don't think the secondary o2 tells the ecu anything other than the fact that you're running leaner/richer than you should be. But, I'm more familiar with the 5th generation accord than your 6th so they might have advanced on the technology by 2000.

I would keep trying to trace the problem through the ignition system. How old are the plugs? This may sound silly, but are they the right plugs and are the tips distorted at all? Might be time to look into a new dizzy.

 

your misfire is likely causing the cat code. If the aftercat o2 reads low efficiency, it will just tell the ECM. If the car is missing and running rich or lean, the aftercat o2 sensor will see a problem, but it won't be the cause. If you are misfiring due to an extremely rich or lean mixture, you would likely also have precat o2 codes.

although, i guess the cat could be slightly plugged and restricting the exhaust flow which could lead to a misfire if too much of the exhaust is staying in the cylinders. i would unbolt the cat and take a look inside it. may be time for a cat.