[bobbecky]

DEF is used to work with the SCR catalytic converter and this system lowers the NOX emissions. The DPF collects diesel soot, and on the GM Duramax, there is a diesel injector the adds fuel into the exhaust stream ahead of the DPF which raises the temperature and burns off the harmful soot. That function requires the truck to be run long enough to build up temps and hard enough to burn off the soot. When this happens, there will be a grey cloud of smoke, not black, that comes out the exhaust, and this is perfectly normal and is necessary to keep the DPF healthy. The best way to keep your diesel truck healthy is 'drive it like you stole it'. They are made to work hard and short distance low speed commuting is really not good for them.