Diagnostic Overview
Detroit SPN 2791 FMI 4 usually indicates an EGR valve actuator circuit voltage below normal or shorted low. The controller is seeing a low signal, open ground path, or short-to-ground condition on the EGR actuator circuit. This can prevent proper EGR valve movement and may lead to derate or emissions faults.
Likely Causes
Short to ground, open circuit, poor ground, loose connector, damaged actuator wiring, failed actuator electronics, or controller circuit problem.
What To Check First
Check actuator power and ground, inspect the harness for rub-through, test continuity and resistance, clean connector pins, and run the actuator test after circuit repairs. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.