Q: P0420 engine error and poor gas/mileage.

Hi there. The catalytic converter on your Honda Accord has two oxygen sensors. One oxygen sensor is positioned in front of the catalytic converter and the other oxygen sensor is positioned in the rear. When the oxygen sensors have a reading that is similar to each other, it is an indication that the catalytic converter is not working as efficiently as it is intended. If the voltage of the downstream oxygen sensor decreases, and begins to fluctuate like the upstream oxygen sensor, it means that the oxygen levels are too high and the Power Control Module will then store the P0420 trouble code. This also tends to impact the efficiency of fuel burning inside the combustion chamber and thus; negatively impact fuel efficiency. The best way to resolve this problem is to have a professional mobile mechanic complete a fuel economy is low inspection; so they can determine if the issue is caused by the O2 sensors or the catalytic converter.