Yup a "Catalyst System Efficiency Below Threshold" P0420. I'm on 32,000 miles on my truck and out of the blue i get this code.I have a test pipe, but I haven't thrown a code for 20,000 miles. Well first off i check if there were any exhaust leaks, but seems fine. So my next assumption would to test the o2 sensor. How would i test that? Oh yeah I rested the light, but it came back after a week. Any other ideas would be great!