What can be causing P0420 engine error and poor gas/mileage?

There are a number of things that can cause a P0420 engine error code, including: -A faulty oxygen sensor -A clogged or restricted catalytic converter -An exhaust leak before the catalytic converter -A problem with the fuel injectors -A problem with the air/fuel mixture If you're experiencing poor gas mileage, it could be due to one of these issues. If you think your catalytic converter may be the problem, have it checked by a mechanic to see if it needs to be replaced. You may also want to check for exhaust leaks before taking your car in for service.