Subaru Repair: Engine Check Light, exhaust gasses, catalytic convertor

Question
So I checked the codes and got P301, P303, and P420. I checked the plugs and they looked fine. Even exchanged plugs 3 with 2 and the codes remained the same. My question is with this combination, being the odd bank is mis-firing could this be the module? Any suggestions of what I should look at?

Thanks!

Answer
Ok ,P0301,P0303 cylinders 1 and 3 misfires as well as p0420.The ignition system should be checked,then the fuel system and lastly the mechanical for the two cylinders misfiring.The burn time for the cylinders should be checked to determine if it is a lean or rich misfire.P0420 is a conditional code telling you that as a result of a misfire the catalytic convertor has been inefficient at cleaning up the exhaust gasses that pollutes our environment.When you examine with the human eye a spark plug you really aren't seeing anything other than the gap of the plug and whether or not it is fouled.It is quite possible that the charcoaling inside of the plug is burnt or broken down causing a decrease in burn time effiency.And while we can use a generic scan tool to  pull codes from the vehicle, tools such as a lab scope and gas analyzer are needed to accurately diagnose the condition.Other than that you can try plugs, wires ,valve adjustment,check injector flow rate,whether or not the head gasket is starting to fail internally.Try to refrain from clearing the codes as it also clears the emission systems monitoring readiness.And you wont really know if you have solved anything until you have performed the drive cycle required to re-establish the emission system monitoring system.