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Hi,
I'm new to this! I've got a 1999 honda civic ex with 107,000 miles give or take. I bought the car in aug then a week ago the cel came on. It's giving me the code p0420 the catalyst system efficiency below threshold (bank1).
Nothing seemed wrong or in immediate repair so I drove the car thinking I could fix it this past weekend. Well 2 days later I was driving and the car decelerated and stopped moving. I would push on the gas and it would act as if in neutral. All the power and the engine was still on in the car. I turned the car off then back on and drover for about a mile and it did this again. So I repeated and it now will only go about 100ft.
It's not running any other codes and the man I bought it from told me the cat was replaced in June but not the o2 sensors. Could the u2 sensors be doing this? My first thought was the transmission! I'm praying it's not.

 
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The code indicates its the CAT An old mechanics trick: Put a gallon of lacquer thinner in a half full tank. Run it on the highway for 150 miles (not all at once necessarily) That will clean the CAT and make the code go away.