Component Location: The SCR (Selective Catalytic Reduction) system relevant to this fault involves multiple components at different vehicle locations. Locations vary by OEM, chassis, and vehicle configuration: - SCR Catalyst: Typically located downstream of the DPF (Diesel Particulate Filter) in the aftertreatment housing. On most Class 8 trucks, this is mounted under the cab or along the frame rail, often on the driver's side. Some configurations use a vertical aftertreatment canister behind the cab. The SCR catalyst is usually the downstream section of a combined DPF/SCR aftertreatment unit. - DEF Dosing Injector (DEF Injector): Mounted in the decomposition tube (mixing pipe) upstream of the SCR catalyst. Location follows the aftertreatment housing position. - DEF Supply Pump: On most platforms, integrated into the DEF tank module (submersible pump inside the tank) or mounted inline on the DEF supply line between the tank and the dosing injector. DEF tank is typically located on the driver's side frame rail, often near the fuel tank or at the rear of the cab. - NOx Sensors: Upstream NOx sensor is mounted in the exhaust pipe before the DPF/SCR system. Downstream NOx sensor is mounted after the SCR catalyst in the outlet pipe. Sensor locations follow the aftertreatment housing routing. OEM variation is significant — verify exact component locations in the OEM service manual or parts diagram for the specific vehicle configuration. Based on: general heavy-duty truck aftertreatment system architecture across Cummins ISX/X15, Detroit DD13/DD15/DD16, and PACCAR MX-11/MX-13 platforms.