Diagnostic Overview
Detroit SPN 2659 FMI 18 commonly points to EGR flow or EGR system performance below the expected level. The engine may not be getting the commanded EGR flow, which can increase NOx emissions and trigger aftertreatment problems. This type of fault often involves carbon restriction or inaccurate pressure/flow feedback.
Likely Causes
Plugged EGR passages, restricted EGR cooler, sticking EGR valve, failed EGR actuator, faulty delta pressure sensor, plugged sensor tubes, intake leak, or exhaust leak.
What To Check First
Compare commanded and actual EGR operation, inspect the EGR valve and cooler for carbon buildup, check delta pressure tubes, and verify boost/exhaust pressure data. Verify final definition in Detroit DiagnosticLink because FMI meaning can vary by engine year, ACM/MCM calibration, and emissions level.