Diagnostic Overview
Detroit SPN 5246 FMI 15 usually indicates an emissions inducement or derate warning at a lower severity level. It means the system has detected an unresolved emissions-related problem and may escalate if the root cause is not fixed. Treat it as a warning that the truck is entering inducement logic.
Likely Causes
DEF quality issue, low DEF system performance, NOx sensor fault, SCR efficiency issue, DPF fault, or other active aftertreatment fault not yet corrected.
What To Check First
Review all active aftertreatment codes, check DEF quality and level, verify dosing and NOx data, and repair the root cause before the inducement becomes more severe. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.