Originally Posted by kaoshen

I had the same problem with my 2000 Honda Civic EX automatic. P0420 COULD be bad 02 Sensors OR it could be the Catalytic converter! I replaced my 02 Sensor and light stayed on. So replaced the Catalytic converter and Cleaned out both 02 Sensors and problem solved!! Civic EX Cats are cheap to replace ($270). Civic LX and DX and CX have catalyctic converter connected to exhaust manifold as ONE PART and that alone costs $960 at hte DEALERSHIP!! Take it form a used car dealer, I've had many of these.