Component Location: The DEF level sensor is integrated into the DEF tank module assembly — a combined unit that typically includes the DEF pump, DEF heater, level sensor, and (on some platforms) a DEF quality/temperature sensor, all mounted as a single assembly into the top or side of the DEF tank. The DEF tank itself is most commonly located on the driver-side or passenger-side frame rail, behind the cab, in a position similar to a fuel tank — often near the rear axle or mid-chassis. On some configurations (particularly day cabs or vocational trucks), the DEF tank may be under the cab steps or mounted on the chassis between the cab and the body. The wiring harness from the DEF tank module runs along the frame rail to the aftertreatment control module (ACM/DCU). Exact location varies by OEM and body builder — always physically identify the DEF tank on the specific vehicle before beginning diagnosis. Based on: OEM parts/service documentation patterns for Cummins ISX/X15, Detroit DD15, PACCAR MX-13, and Navistar A26 platforms.