Component Location: The DEF quality sensor is mounted directly into the DEF (Diesel Exhaust Fluid) tank — typically on the top or side of the DEF tank via a threaded bung. On most Class 8 trucks, the DEF tank is located on the driver-side frame rail, behind the cab, forward of the rear axle (often near the fuel tank or mounted separately on the frame). The sensor probe extends down into the DEF fluid inside the tank. On some configurations, the DEF quality sensor is integrated into the same sending unit assembly as the DEF level sensor and DEF temperature sensor (a multi-function module mounted to the tank). Location varies by OEM and vehicle body manufacturer — on some vocational trucks, the DEF tank may be mounted on the passenger side or behind the cab in a different position. Always physically locate the DEF tank on the specific vehicle before attempting sensor removal. The DEF quality sensor connector is typically accessible from above or the side of the tank. Based on: general DEF system architecture across multiple platforms, technician field reports, and OEM service documentation patterns. Verify specific location for the vehicle at hand.