Component Location: The outlet NOx sensor (SPN 3226) is located downstream of the SCR (Selective Catalytic Reduction) catalyst — it measures NOx after the urea/DEF reaction has occurred. Physical location varies by OEM and vehicle configuration: - Cummins ISX15 / X15: Typically mounted in the exhaust outlet pipe exiting the SCR catalyst, which is usually part of the aftertreatment assembly on the right side of the engine or the vehicle frame rail. On chassis-mounted aftertreatment systems, the SCR outlet is often near the DPF/SCR module at the back of the engine or mid-chassis. - Detroit DD15 / DD13: The SCR outlet NOx sensor is located at the tailpipe-side outlet of the SCR module in the single-module aftertreatment assembly, typically mounted on the right-side frame rail behind the cab or in the engine tunnel area on Class 8 trucks. - PACCAR MX-13: Similar to above — sensor located at the exit of the SCR catalyst within the aftertreatment module. IMPORTANT: Do not confuse the outlet NOx sensor (after SCR) with the inlet NOx sensor (before SCR, after turbo/DPF). The outlet sensor is the one closer to the tailpipe. The connector and harness typically route back toward the ECM/ACM along the frame rail or firewall. Exact mounting location varies by chassis OEM (Peterbilt, Kenworth, Freightliner, International, Volvo, Mack) — verify against the specific vehicle's aftertreatment layout. Based on: OEM aftertreatment system architecture documentation + technician field experience across multiple platforms.