my truck throws the p0420 code after about 30 miles .put new oxygen sensor in converter.same results. took old oxygen sensor that came from converter and changed out the front one . exact same results. anybody ever went through this and found out what is wrong. the truck has 181,000 miles on it and runs good except for a little rough idle while engine is cold, thanks

 

Your passenger side cat is shot or you have an exhaust leak on that side. There was nothing wrong with your O2 sensor, it was doing its job.

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)