If I am sure, P0420 is the Intake Manifold by the firewall, that is not working right. P1131 is the O2 sensor that need to replace. Usually the 3rd *main* cat does not go bad as fast as Bank 1 and 2 does. You can try to clean it and replace the O2 sensor first to see if it will work, however, if the same code still came out between a week to a month, then you need to replace the Intake Manifold Catalyst system. Here is the part number for the cat and gasket:
F8RZ-5G232-BC Intake Manifold Catalyst System - $369.55
XW4Z-9448-AD Gasket - $15.18
