Updated diesel emissions regs mandated improvements in NOx (Nitrogen Oxide) emissions. Like Ford, GM went the SCR (Selective Catalytic Reduction) route and uses a 5.3-gallon DEF tank (Diesel Emissions Fluid, which is 32.5-percent urea and 67.5-percent deionized water). The DEF is injected into the hot exhaust and breaks the NOx down into harmless nitrogen gas and water vapor. The tank, filled from under the hood, will last approximately 5,000 miles, and the PCM will give you plenty of warnings to fill the tank. Should you ignore them all and run the tank dry, the engine will go into a limp-mode-type state until you refill it. DEF is expected to cost two to three dollars a gallon initially, but will likely go down as the aftermarket kicks into high gear.