Code P0402 on the 2024 Nissan Frontier indicates excessive Exhaust Gas Recirculation (EGR) flow has been detected. This typically means the EGR valve is stuck open or allowing more exhaust gas into the intake manifold than the ECM commanded.
EGR System Function
The EGR system reduces nitrogen oxide (NOx) emissions by recirculating a portion of exhaust gas back into the intake manifold. This dilutes the intake charge and lowers combustion temperatures, reducing NOx formation. The ECM precisely controls EGR flow based on engine conditions.
Excessive EGR Effects
When EGR flow is excessive, the engine receives too much exhaust gas, diluting the air-fuel mixture excessively. Symptoms include rough idle, hesitation, poor acceleration, decreased fuel economy, and potential stalling. The engine may feel like it's running on too few cylinders.
Common Causes
Carbon buildup preventing the EGR valve from closing completely is most common. The EGR valve motor or actuator may have failed in the open position. A faulty differential pressure sensor can misreport flow levels. Wiring issues or stuck EGR position sensor can cause false readings.
Diagnostic Strategy
Command the EGR valve closed and verify it actually closes using visual inspection or position sensor data. Check for carbon buildup in the valve and passages. Test EGR valve motor operation. Verify differential pressure sensor readings are accurate.