What does the p0420 code mean?
(Catalyst below threshold)
Look at the wiring to downstream O2 sensor, possible bad O2 sensor, or the catalytic converter isn't working like it should be. Also, possible exhaust leak upstream from catalytic converter.

What do the spark plugs look like? (Oil fouled, carbon fouled, or expected light tan)

Is your engine burning or leaking oil? (1 to 2 quarts a week isn't "a little bit")

Take care,
ChrisS