Diagnostic Overview
Mack fault code SPN265918 generally points to EGR flow or EGR mass flow below the expected range. The ECM may not be seeing enough EGR flow when commanded, which can raise NOx emissions and trigger additional aftertreatment faults.
Likely Causes
EGR valve stuck closed; plugged EGR cooler; carbon restriction; failed EGR actuator; plugged pressure tubes; sensor fault.
What To Check First
Run an EGR command test, inspect the EGR valve and cooler, clean or inspect pressure tubes, and compare live flow data.