Hey all, I'm chasing down a problem on my daughters 2000 civic.
We've had it for a year and a half or so. Initially I replaced the exhaust manifold & catalytic converter & o2 sensor cause the manifold was cracked and it was throwing a P0420 code. It also didn't want to start in the cold.
I ended up putting new plugs & wires in it also. Well... it's cold in MN again and it's doing the same thing.

All of my research for P0420 says to replace the cat and/or o2 sensor which I did just over a year ago. What thoughts do any of you have???

 

welcome to the forum.. you wrote that you did change a O2 sensor, there are 2 ea. also must be Honda sensor no after market even if aftermarket is same make. check wiring, check timing, and eng temp sensor may cause code P0420

 

On Honda vehicles, there are two O2 sensors for each bank
One is up stream and is a air fuel ratio sensor type
The other is downstream and is the one that checks the efficiency
ACmech52 is right. Honda vehicles are sensitive and need Honda OEM O2 sensors.

If you have Honda sensors and still have P0420 then might want to check the wiring.
The next thing to do is to put injector cleaner in the gas.
Some people use stuff like Cataclean to try to clean the cat.
Don't know if it really works.