Diagnostic Overview
Mack fault code SPN265916 generally points to an EGR flow or EGR mass-flow condition above the expected range. The ECM is seeing EGR behavior that is too high for current engine load and operating conditions. Excess EGR can cause smoke, rough running, poor boost response, and soot loading.
Likely Causes
EGR valve stuck open; incorrect EGR differential pressure reading; plugged sensor tubes; boost leak; exhaust pressure issue; wiring or sensor fault.
What To Check First
Inspect EGR valve operation, compare commanded versus actual EGR flow, check pressure sensors and tubes, and verify intake/exhaust integrity.