SCR Inlet DEF Pressure Low

Detroit Diesel fault DD-4334-1 (SPN 4334 FMI 1) indicates: SCR Inlet DEF Pressure Low. It is a serious-severity fault and is commonly seen on DD13/DD15/DD16 engines in Freightliner and Western Star chassis.

It depends on severity. Critical and serious faults trigger derate or engine shutdown — do not continue driving. Moderate faults usually allow continued operation short-term but schedule repair to prevent cascading aftertreatment damage.

Estimated repair cost is $1,200-$3,800. Sensor/harness fixes fall at the low end; turbo actuator, injector, aftertreatment component, or head gasket repairs fall at the high end. Detroit DDDL software and sometimes factory parameter reset are required for proper reset.

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.