“P0420 is a catalyst efficiency code. Given the vehicle sat for a couple of years, I would tale a look at the upstream and downstream O2 sensors and do the other P0420 diagnostics. See this ETCG link: https://www.youtube.com/watch?v=9VZ5K8n5jj0”
That’s a good video tutorial, Dave. We’ll just finish installing the parts from napa and 4Wheelonline and we’ll start working on buddy’s Silverado having the code.