Exhaust Gas Recirculation Flow Insufficient Detected
Your engine's exhaust gas recirculation (EGR) system isn't flowing enough exhaust back into the engine to reduce emissions. Think of it like a water recirculation pump that's not pushing enough water back through the system.
The ECM monitors EGR flow by measuring intake manifold pressure changes and comparing them to expected values when the EGR valve opens. It uses oxygen sensors and mass airflow data to calculate actual recirculated exhaust volume. When measured flow falls below calibrated thresholds, the fault is triggered.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| EGR Flow Rate | 4-8% of intake mass airflow | Less than 2% flow detected during EGR operation |
| EGR Valve Position | 0-100% commanded opening correlates to measured flow | Commanded position mismatch with actual flow |
Code P0402 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P0402 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.