2003 Mitsubishi Outlander – Running Rich

I am having some problems with low fuel mileage on my 2003 Mitsubishi Outlander LS – 131000 mi. I have done the following within the last three months;

Changed fuel filter
Changed air filter
New Bosch Iridium Plugs
Seafoam through vacuum line
New front o2 Sensor
Transmission flush
Cleaned EGR valve
Cleaned intake manifold
Replaced small vacuum hose
New PCV Valve
Oil change (synthetic)
Cleaned MAF, MAP and IAC.
Coolant temperature sensor

The following things have been checked as to working;

Battery
Wires/Coils
Fuel Pump
Engine Compression

I am not sure what there is left to do other than start changing sensors. I am getting around 12 MPG. Check engine is on with P0420 code however the catalytic has been examined and is working. A couple of observations are an exhaust smell(rich) when parked after opening doors, idle is rough for the first minute when the car is started in the morning. At times when I let off the gas it seems that the car is slowing down quicker than it should.

I have cleaned the injectors using a cleaner in the tank plus removing them and running carb cleaner through them. MAF sensor has been suggested to change but I am not getting any codes with it. I disconnected it and noticed a severe degrade in performance, since then I reconnected it. I suspect it is a sensor but before I start buying I would like some direction.

Thank you for your input.