Diagnostic Overview
Detroit SPN 1172 FMI 3 usually indicates a turbo/air-system pressure sensor circuit voltage above normal. The ECM is seeing a high signal from a sensor used for boost or air-management calculations. This can create false high-boost readings or lead to derate.
Likely Causes
Short to voltage, failed pressure sensor, connector corrosion, damaged harness, incorrect sensor reference voltage, or controller input fault.
What To Check First
Check the sensor reference, signal, and ground circuits; inspect the connector; compare key-on pressure readings to barometric pressure; and replace the sensor only after the circuit tests correctly. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.