Diagnostic Overview
Detroit SPN 2629 FMI 4 usually indicates an air-management or turbo-related sensor circuit voltage below normal. The ECM is seeing a low signal that may prevent accurate boost or airflow calculation. This is typically an electrical diagnosis first.
Likely Causes
Short to ground, open signal wire, failed sensor, damaged harness, poor connector contact, low reference voltage, or controller input issue.
What To Check First
Identify the exact sensor in DiagnosticLink, inspect the harness, test 5-volt reference and ground, compare key-on readings, and repair circuit faults before replacing major turbo components. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.