I purchased my 2011 Traverse LTZ, August 2014 with 42,000 miles. Today, it has 171,424 miles. P0420 has thrown other codes which disappeared when I did the work listed below.
1. Installed new spark plugs (ACDelco 41-109 Iridium Spark Plugs) 19.99
2. Installed new ignition coils (D515 Ignition...