Diagnostic Overview
Volvo fault code P0402 usually indicates excessive EGR flow. The ECM is seeing more recirculated exhaust gas than expected, which can cause smoke, rough running, low power, soot loading, or unstable combustion. This can be caused by a valve that is leaking or stuck open, or by incorrect sensor feedback.
Likely Causes
EGR valve stuck open; leaking EGR valve; incorrect EGR pressure sensor data; plugged sensor tubes; actuator fault; wiring issue.
What To Check First
Check commanded versus actual EGR position, inspect the EGR valve for sticking, verify pressure sensor data, and inspect sensor tubes and connectors.