SCR NOx Catalyst Efficiency Below Threshold

P20EE on a MaxxForce 11 (10.5L) (2008-2014) indicates: SCR NOx Catalyst Efficiency Below Threshold. This is a known fault code on the MaxxForce engine family and typically points to the causes listed above.

Only drive short distances to reach a repair facility. Continued operation may cause further damage or progressive derate.

Repair cost is typically $500-$6,500 depending on the underlying cause and shop rate. Many MaxxForce faults (especially EGR cooler, head bolt, and VGT repairs) are major expenses that drove fleet owners to repower with Cummins or retire these trucks early.

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.