I've searched all around and I'm stumped. Two summers ago, I replaced the cat with a non-OEM one. Not a performance one, just a normal cheaper, non-OEM cat. Both O2 sensors were replaced at the same time. The car is a 1999 Civic LX, with 268,000 miles.

About a month ago, the CEL came on and it was the dreaded P0420. About a week later, the CEL started to flash (indicating a misfire?). The car ran fine while it was flashing, other than a little rough idling. It only did this one day, I've been driving it for a month and it only happened that one time. I keep clearing the code, but after a few days-a week it comes back on. The gas mileage has dropped significantly too. I usually would get ~300 miles/tank on mixed city and interstate, now I'm lucky to get 250 miles. The car runs fine, but occasionally the idle drops extremely low. Once, it dropped so low at a red light it actually stalled out.

Anyone have any insight at all?

Thanks!
Colin

 

Pull the codes there may be something else in there now. Misfiring while driving is usually bad plugs or wires. Misfiring only at idle is often the valve adjustment. Stalling out you should clean the IACV. Check timing with a light. Wrong timing will reduce power and gas mileage without throwing codes.

 

you can use a mechanical o2 simulator to get rid of the p0420 code unless the problem is the o2 sensor itself and not the cat. megan racing sells a o2 simulator.