OK, I have replaced the primary o2 and made sure there were no exhaust leaks. I replaced the exhaust gasket just after the cat on my y7 as well. The P0420 code still comes back. its usually on part throttle. Any more suggestions??? The car doesnt get that bad a gas mileage. About 29-30 at 75 to 80 mph highway.

 

A P0240 says it's a Catalyst System Efficiency Below Threshold (Bank 1). Possibly it's your catalytic converter? Is the manifold cracked?

 

Okay, here is the cure to my problem. I looked on the bosalusa website and it does say on the replacement cats p0420 codes that it is a common thing on those civic's, esp those with high miles. The cure for it is a faulty or failing ignitor. I replaced the entire distributor on my car since it is the original one with over 200k miles.

BINGO.............. CEL for p0420 is gone. Evertime I reset the computer, the CEL would come back on after a few days, even faster with ac running Like minutes.

 

That doesn't make sense at all.

 

Poor Connections at Igniter Terminals May Cause P0420
Honda Service Bulletin 05-027 dated June 10, 2005 states that certain Honda Civic applications may be prone to developing poor connections at the igniter terminal. This can result in a DTC P0420 or the engine pings under load. DTC P0301 – P0304 may or may not also be set indicating a cylinder misfire.
Prior to replacing the catalytic converter, it is recommended that the wires from the distributor subassembly be crimped and double-check to confirm that no cylinder misfires are present.

Copied from Bosalna.com

This may not help everyone but it helped me. This was after a few posts where peeps kept saying Replace 02 sensor................ And after swapping 3 different dx cats and a new entire cat back exhaust due to rusting out issues, and replaced primary o2 sensor 3 times with new Denso units as well.