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My 02 civic has been driving fine with no problems since getting it, one day driving for about 1hr and in stop-and-go traffic, I noticed that when I pushed the gas it was hesitating severely to go then instantly shut down to a consistent 2k rpm. I pulled off to it then died and I waited for a ride. Once he got there an hour later or so I started it back up just fine and decided to try to drive it home which did just fine. The problem still persists as I drive it for a longer period it gets very sluggish and doesn't wanna accelerate. The only code it's thrown is p2000 nox threshold efficiency below. I went ahead and removed the catalytic converter today and didn't seem to help. I also ordered an upstream O2 sensor today and will be putting it in to see if it helps. Any other ideas?