P0420 CODE

2003 Dodge Caravan • 100,000 miles

My van's check engine light is on. I got the code read and it shows a p0420. I checked the book and it says it is Catalyst System Efficiency Below Threshold (Bank 1). The van idles fine, when I go up hills (in Colorado) it feels like it lacks power. Do you think it is an O2 sensor, or is it the catalytic converter? What is a way that a layman like myself can tell?
April 12, 2013.

The code means the oxygen sensor after the catalytic converter is reading nearly the same as the one in front. That means no change is taking place in the composition of the exhaust gas as it passes through it. The catalytic converter is not doing its job. The oxygen sensor is working properly because it reported the condition to the Engine Computer.

Apr 13, 2013.