CODE PO401 : Insufficient EGR Flow

The Exhaust Gas Re circulation (EGR) system is designed to reintroduce exhaust gas into the combustion chambers, thereby lowering combustion temperatures and reducing the formation of Oxides of Nitrogen (NOx). The amount of exhaust gas that is reintroduced into the combustion cycle is determined by several factors, such as: engine speed, engine vacuum, exhaust system back pressure, coolant temperature, throttle position. All EGR valves are vacuum operated. The EGR vacuum diagram for your particular vehicle is displayed on the Vehicle Emission Control Information (VECI) label. The EGR system is Differential Pressure Feedback EGR (DPFE) system, controlled by the Powertrain Control Module (PCM) and composed of the following components: DPFE sensor (also referred to as the back pressure transducer), EGR Vacuum Regulator (EVR) solenoid, EGR valve, and assorted hoses. The P0401 code means that OBD detected an insufficient amount of EGR. Symptoms There may be drive-ability issues such as pinging ( pre-ignition knock , similar to cheap gas) when the vehicles engine is under heavy load or the vehicle is climbing a steep grade. There may also be other symptoms. Causes A code P0401 most likely means one or more of the following has happened: The DPF (differential pressure) sensor is faulty and needs to be replaced There is a blockage in the EGR (most likely carbon buildup) The EGR valve is faulty The EGR valve may not be opening due to a lack of vacuum Possible Solutions In fixing this code, it is quite common for people to just replace the EGR valve only to have the OBD code return. The EGR valve is not always the cause. If you replace all 3 components the like is all but guaranteed to go away . Or you can test each component . Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's. There should be a noticeable difference in RPM's with the EGR open Clean out the EGR valve to remove deposits Replace the DPF sensor Replace the EGR valve