NOx Outlet Sensor (SCR Downstream) Data Erratic

Volvo/Mack code MID128-PID3226-FMI2 (NOx Outlet Sensor (SCR Downstream) Data Erratic) is a serious-severity diagnostic trouble code set on D11/MP7, D13/MP8, and D16/MP10 diesel engines in Volvo VNL/VNR and Mack Anthem/Pinnacle/Granite/Titan trucks. It indicates a fault in the NOx outlet sensor system and should be read with Volvo Premium Tech Tool (PTT) or Mack Guided Diagnostics.

Short distances may be acceptable to reach a repair facility, but the fault will escalate to derate (typically 55 mph then 5 mph on aftertreatment faults). Do not defer diagnosis beyond a shift.

Typical repair falls in the range $400-$900. Sensor/wiring repairs are at the low end; EGR cooler, NOx sensor set, DEF pump, injector, or head work runs at the high end. Volvo PTT or Mack Guided Diagnostics is normally required for calibration and parameter programming.

California's heavy-duty OBD regulation requires the system to flag a NOx-converting catalyst when its conversion capability drops enough to push NOx emissions more than 0.4 g/bhp-hr above the applicable standard (Title 13 CCR 1971.1, section (e)(6.2.1)) — so on a 0.2 g/bhp-hr engine the trip point is around 0.6. Catalysts in series may be monitored individually or as a group, which is why a code can point at the aftertreatment as a whole rather than at one brick. This threshold comes from the heavy-duty OBD regulation, which covers 2010 and later heavy-duty engines. If your engine is in a pickup at or under 14,000 lb GVWR it is certified under the separate medium-duty rule instead, so treat this as the design intent rather than your engine's exact limit.