Diagnostic Overview
Mack fault code P0402 indicates excessive EGR flow. The ECM is seeing more EGR flow than expected, which can cause rough running, smoke, low power, poor turbo response, and soot-loading issues. This may be caused by an EGR valve that is leaking or stuck open.
Likely Causes
EGR valve stuck open; leaking EGR valve; incorrect pressure sensor reading; plugged sensor tubes; wiring fault; air-handling imbalance.
What To Check First
Check commanded versus actual EGR position, inspect the valve for sticking or leakage, and verify pressure sensor readings.