I have a 2000 civic EX with 86,000 mi. I had a cec P0420 on about 18 months ago, took it to the dealership and they said the cat was bad. They replaced it as it was still under warranty (71,000 mi). About a month ago the cec P0420 came on again, saying the cat is failing. I took it back to the dealership and they said it was bad again, but it only had a 12 month warranty, so this ones going to be out of my pocket. What could have caused the cat to fail within 15,000 miles? The acceleration and gas mileage is horrible, and it sounds like it's spitting when you accelerate. Any ideas?

 

Have you looked at your O2 sensor? That is listed as one of the causes of that code.

 

The dealer said if it was an o2 sensor, it would have its own code.

 

spitting? is there smoke coming out of the air intake, or anywhere else in the car?

with my clogged cat it would sound like it was spitting and the only way to keep it running was to hold on the gas. unbolted the cat and it ran fine (loud).