Pattersonca

I have a 1999 Honda Civic DX door sedan with the 4 cylinder engine.
A couple days before I had to smog my car the check engine light came on. The code P0141 for downstream sensor came on. We replaced the downstream sensor and cleared the code. Drove the car and the check engine light came back on with the same P0141 code 30 minutes later. We cleared the code and drove the car for a day and the light did not come on. I had the car smogged and it easily passed. The HCs were between 9 and 13 at idle and higher rpm. The following day, the check engine light came on again. Now it is showing code P0420 for bad cat. The car has 340K and the cat was replaced and has about 120K. I am thinking the problem could be with the O2 wires or the ECU? Not sure what to do. I cleared the code again to see what would come up and the light has not come on since driving the car an hour this morning.

Any suggestions would be greatly appreciated