99 Honda Civic "PO 420"

A while back I had a bad oxygen sensor caused by a cracked exhaust manifold ? I replaced both and all was good for about 6 months. Now I am getting a ?PO-420? code (Catalyst system efficiency below threshold), any ideas what I should do here? Thanks folks!

I suppose it could be the other Oxygen sensor. It could also be that your new catalytic converter isn’t performing to specifications. Is it covered by a warranty?

How did the cracked exhaust manifold cause the (front?) oxygen sensor to go bad? (The question mark is a real question indicator.)
The oxygen sensors’ outputs are compared to a base value, and, they are compared to each other. From this comparison (whether the oxygen sensors are correct, or not), the engine computer decides upon catalytic converter efficiency.
It couldn’t hurt to change the 10 year old oxygen sensor. If that doesn’t do it, further tests need to be done before just changing the catalytic converter.
This information will help you with the problem: http://www.obd-codes.com/p0420

I have seen this before. Because the exhaust manifold (which is also the catalytic converter) got hot enough to crack, it also fused the top Oxygen sensor in place so that it won’t come out no matter how hard you try.

Could be simply water in one of the O2 sensor connectors. Try cleaning them and reconnecting them.