Question

2001 Honda Accord EX with engine code P0420, lower power, lower mpg

Answer 1

P0420 Catalyst system efficiency below threshold

Fuel trim that is outside manufactuer specification rapidly degrades the catalytic convertor, worn sparkplug , plugged air filter , ignition timing, even a failing 02 sensor , these will rob the vehicle power and lower the MPG. You may just need a full tune up.