Diagnostic Overview
Mack fault code P0401 indicates insufficient EGR flow. The ECM commanded EGR operation but did not see the expected change in airflow, temperature, or pressure. Low EGR flow can increase NOx emissions and may contribute to aftertreatment or derate complaints.
Likely Causes
Plugged EGR cooler; carbon buildup; stuck closed EGR valve; failed EGR actuator; plugged differential pressure tubes; intake/exhaust leak.
What To Check First
Inspect EGR valve movement, EGR cooler restriction, pressure tubes, and live EGR flow data during a commanded test.