Diagnostic Overview
Detroit SPN 1172 FMI 21 commonly relates to turbo compressor outlet, boost, or air-system pressure rationality depending on calibration. The ECM may detect that the measured air-system value does not agree with expected turbo and engine operating conditions. This can cause poor performance, smoke, or derate.
Likely Causes
Boost pressure sensor issue, charge-air leak, intake restriction, exhaust leak, turbo actuator fault, sensor contamination, wiring problem, or calibration-specific air-system rationality failure.
What To Check First
Compare manifold pressure, barometric pressure, turbo speed, and commanded boost; pressure-test the charge-air system; inspect sensor connectors; and check for related turbo actuator faults. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.