P0420 Catalyst System Efficiency below Limit - Bank No 1.
Bank 1 is closest to the firewall. I believe this code will be caused by a bad reading from the downstream sensor, which is easiest to reach from under the car. Could also be a bad pre-cat. You could swap the downstream sensors between the front and rear manifolds to see if the problem moves to the other bank.
http://www.contour.org/FAQ/FAQ.cgi?s=probs&display=checkengine
