Diagnostic Overview
Detroit SPN 411 FMI 4 commonly relates to EGR differential pressure or EGR delta pressure sensor voltage below normal. The ECM uses this signal to estimate EGR flow. A low-voltage fault can make the engine believe EGR flow is incorrect, causing emissions warnings, rough operation, or derate.
Likely Causes
Failed EGR delta pressure sensor, short to ground, open circuit, plugged sensor tubes, corroded connector, damaged harness, or moisture/soot contamination in pressure lines.
What To Check First
Inspect and clean the delta pressure tubes, check the sensor connector, test the signal voltage and ground, compare live pressure data, and verify EGR flow after repair. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.