Component Location: The intake NOx sensor (also called the upstream or pre-SCR NOx sensor) is mounted in the exhaust stream upstream of the SCR catalyst, typically after the diesel particulate filter (DPF) and before the DEF injection point. Physical mounting location varies by OEM and chassis configuration: on most Class 8 trucks, it is located on the aftertreatment assembly mounted on the frame rail (commonly driver or passenger side depending on OEM), on the inlet pipe or inlet face of the SCR housing. On some configurations it may be mounted on a mid-pipe section between the DPF outlet and the SCR inlet. The sensor body is externally similar to a bung-mounted oxygen sensor but with a multi-pin electrical connector (typically 4–6 pins depending on OEM). NOTE: Exact mounting location varies significantly by OEM and vehicle configuration (frame-mounted aftertreatment vs. cab-behind-engine). Always identify the specific aftertreatment layout on the vehicle before locating the sensor. The NOx sensor heater and ECU module (sometimes a separate control unit mounted near the sensor) may also be present. Based on: general industry aftertreatment layout knowledge, Cummins/Detroit/PACCAR platform documentation, and field technician descriptions.