2007 Endeavor P0421
 

Went to get car inspected and they said the service engine light was a direct fail in Texas. The code is P0421 Warm up catalyst efficiency below threshold. The car has under 40,000 miles and that sure seems like very low miles for a Cat converter to fail. Would this most likely be a O2 sensor? if so is it hard to replace myself? Any help would be greatly appreciated.