Check engine light on my 2004 ford taurus fault code P0420?

I ran the code running a generic OBD2 scanner and I got P0420-ford saying

Type- Powertrain Auxiliary Emissions Controls ISO/SEA Controlled
Description- Catalyst System Efficiency Below Threshold
Location- Bank 1

If anyone knows what that means can you please explain

O2 sensor or clogged catalytic converter. Your local auto parts store should be able to tell you which o2 sensor it is since there should be 4 if I remember correctly. Bank 1 and Bank 2, upstream and down stream sensor.

If it runs as good as it ever did, ignore it.

Look it up on ford forum