Learn how to change the downstream (after the catalytic converter) O2 sensor. My Civic Ex was throwing check engine code P0420, which prompted this install. I was able to figure out that it was the O2 sensor rather than the actual cat because the CEL intermittently was coming on and off over about a weeks time. Like I said in my other Air/Fuel/O2 sensor video, Denso parts are cheaper than OEM, but they are basically OEM since Denso makes many parts for Honda's straight from the factory.
Honda part 36532-PLR-A11 Denso part 234-4220