I have an 03 Mercedes C320 4 door sedan and I had a CEL come on with error code P0172. I have noticed a visible reduction in fuel mileage as well as power. In the last year I had the spark plugs replaced, O2 sensors and also the MAF sensor replaced. How can I diagnose the root cause of this problem ? I have a dashcmd OBD scanner as well. Would like to learn and understand before I take it to a mechanic shop.