A common belief on “chipped” diesel engines is, if your fuel economy is improved by using an aftermarket “chip”, your emission levels drop also.  This is not necessarily true.  From the manufacturer, your diesel engine is made to meet certain goals for that particular motor; fuel economy, power, reliability and emissions to name a few.  “Chipping” your diesel powerplant will change some or all of these categories, sometimes for the better, sometimes for the worse.  Some chips claim up to 5 MPG’s better than stock numbers, but because the engine control management (ECM) computer is being told to back off on fuel delivery in certain circumstances, the engine is in a “lean” state and is likely running hotter, which can create more emissions.  In places that have smog control standards, like California, “chipping” your diesel is illegal.  This is because there is lack of proof that the device does not increase emissions, and until aftermarket chip manufacturers can prove otherwise to the Air Resources Board in the U.S. , it will likely stay that way.