Diagnostic Overview
Detroit SPN 2791 FMI 3 usually indicates an EGR valve actuator circuit voltage above normal or shorted high. The controller is seeing an electrical signal that is higher than expected on the EGR actuator circuit. This is usually an electrical diagnosis before it is a mechanical EGR diagnosis.
Likely Causes
Short to voltage, damaged harness, corroded connector, failed EGR actuator electronics, incorrect supply voltage, or controller output fault.
What To Check First
Inspect the EGR actuator harness for rubbing or heat damage, check connector pins, test voltage on the actuator circuit, and confirm power and ground before replacing the actuator. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.