Diagnostic Overview
Volvo fault code P0401 usually indicates insufficient EGR flow. The engine controller commanded exhaust gas recirculation but did not see the expected pressure, airflow, or temperature response. This can increase NOx emissions, create driveability issues, and trigger related aftertreatment or derate faults if ignored.
Likely Causes
Plugged EGR cooler or passages; sticking EGR valve; failed EGR actuator; restricted EGR differential pressure tubes; intake or exhaust leak; sensor fault.
What To Check First
Inspect EGR valve operation, check EGR cooler restriction, clean or inspect pressure tubes, review EGR differential pressure data, and check for boost or exhaust leaks.