DTC P0401: Exhaust Gas Recirculation (EGR) Insufficient Flow: General Description

The exhaust gas recirculation (EGR) valve, which is controlled by the engine control module (ECM)/powertrain control module (PCM), is opened and the exhaust gas flows from the exhaust manifold through the EGR valve and the intake manifold and the EGR passage. The exhaust gas is circulated into the air/fuel mixture to lower combustion temperatures, thus reducing oxides of nitrogen (NOx) emissions. The EGR flow is inspected as follows. The EGR valve is closed during deceleration with the throttle valve fully closed. Then the ECM/PCM fully opens the EGR valve. After set time, the ECM/PCM computes the ratio of the present EGR flow to the normal EGR flow by monitoring the fluctuation of the intake manifold pressure when the EGR valve is fully opened and when it is fully closed. If the EGR flow rate is lower than normal, a malfunction is detected and a DTC is stored.