Hey all!

I have a manual 2002 Honda civic LX, stock as far as I know. I'm having a little trouble figuring out what exactly is wrong with my civic. when I, A: travel 65+mph for 5-20 minutes, or B: rev/switch gears at more than 3k revs, the CEL comes on (P0420) and it starts slowing itself down. When I restart the car, it is completely fine until I do it again, but the CEL stays on for a few more miles and then turns off. I have new 02 sensors, and a new exhaust manifold. I'm wondering if the manifold either has a small hole somewhere, or the gasket got bent upon installation, because I can sometimes smell exhaust in the cabin when I idle at lights and such. I don't know if this is related, but when I idle, about every 20-30 seconds, the revs drop down to near 0 for a second, and then goes back to normal.

Any help y'all could throw my way would be greatly appreciated, thank you!

 

check the other thread about P0420 codes, read the bit about freeze frame data with a scan tool. will tell you what the sensors see. report back.

idle stumble shouldn't be related to the P0420 code. also, I don't think P0420 is a limp mode code. the slowing down part is a curiosity. I have an EX (Si) model and at highway speed I'm well over 3K all the time, it would drive me insane to have it bogging like that.

are you sure about the code? does the CEL flash or steady on? is the car jerky at all when doing this slowing down business? it sounds like you have a miss... like a bad coil or bad plug. When was the last time you pulled the plugs for an inspection?

 

yes, I've tested the code several different times, and they were the same each time. the CEL is steady on(until it goes off a few miles later), but does not flash. The car doesn't jerk, it only starts gradually slowing down the RPMs, no matter how hard I pushed the gas. It even started to overheat one time when it first started doing this, because I was trying to push the gas hard. I've had this car just under a year, but every time before I buy a used car, I have it checked out by firestone. The car runs completely smooth until it pops the code and limps, or else my first thought would be the coil/plugs too. It does however sputter like it's not getting enough gas for about 3 seconds first thing in the morning when it's completely cold, and then it runs perfectly the rest of the day. I don't know if this is relevant, but my CEL was on 24/7 with the code before, and then I changed the 02 sensors and exhaust manifold, then the CEL went off, and only comes back on with my current problem. I'll probably go ahead and replace the spark plugs to be on the safe side, since they're cheap and I don't know the history of the car.
P.S. I have 167k miles on it.

Thanks!