Hi I have a 2004 Chevy 2500 crew cab duraMax diesel and the check engine light has come on.I ran a check on it and came up with 3 codes 1. P0401 insufficient exhaust gas recirculation flow detected 2. P0101 mass or volume air flow circuit range/performance problem and 3. P0404 exhaust gas recirculation circuit range/performance. Anyone have any ideas on what could be causing this and how to fix it would be greatly appreciated. Lance