What does the 2009 chevy traverse p0420 code mean?

If You are driving Your 2009 Chevy Traverse and You get a check engine light on the dash the first thing You should do is try to get off the road as soon as possible to prevent extra damage to Your car. Once You are stopped You can use an OBD II scan tool to read the code that is thrown by the engine. If You have a code P0420 in the ECU then that means that Your O2 sensors are not working properly. I recommend that You check the wiring to the sensors and make sure that they are free from debris. The O2 sensors in Your car measure Your exhaust gasses and help make sure that Your engine runs smoothly so they should be replaced if they fail.