check engine light on 2006 Lincoln Navigator

P0420 trouble code. Can reset, but comes back on in 2 to 4 days. Trying to figure out if it is a 02 sensor or the catalytic converter.

Asked by for the 2006 Lincoln Navigator
To accurately check the O2 sensors will requires a 2 channel lab scope and you need to know the correct / incorrect wave forms of the good/bad sensors under different conditions.
I'd leave this to a professional - otherwise you might spend $$$ for unnecessary parts.