DEF = Diesel exhaust fluid, (Diesel exhaust fluid is an aqueous urea solution made with 32.5% urea and 67.5% deionized water) necessary for any diesel made 2010 or later. We have the Mercedes chassis on which the Gemini is built, uses DEF, but not that much, at least when underway. If you are in stop and go traffic, or idle a lot DEF consumption will skyrocket. DEF can be added anytime, most vehicles do have some sort of display. On mine, one of the buttons near speedometer, shifts the fuel level display to DEF level display.
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