Your 2005 pacifica has an error code P0420 Is this the catalytic converter or an oxygen sensor How much should it cost to replace converter if necessary?

P0420 is a code involving O2 readings being off specs. It doesn't always signal a bad cat. Can be a bad O2 sensor. P0420 is bank 1, meaning up front sensor. Sometimes the sensors go bad due to not tuning the vehicle up and keeping things running smoothly. A misfire/bad plug/bad wire can result in poor fuel economy, poor engine performance and over time clog the O2 sensor(s). With the relative young age of your vehicle, I'd suspect the upstream O2 sensor. Converter don't go bad too easily and are very expensive.