Some history...in May 2013, the main catalytic converter unit that is part of the exhaust manifold went bad and was replaced. Two weeks ago, the secondary cat and pipe were replaced after it started metallic rattling noise (probably from core breaking apart). Secondary O2 sensor was replaced at the time, too. Now we're getting a P420 cat code check engine light. Plugs and PCV were changed 47K miles ago and air filter was changed this January. Daily driver 15 miles one way to work....mostly highway. Is the main cat bad again now after 23000 miles? Or could it be something else? Do not know if main O2 sensor was changed with main cat. Car gets good gas mileage (ave 27-30 but will get 33+/- on trips) and runs fine. Uses 1 to 1.5qt oil in 5000 miles. 147000 miles. Suggestions?