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DPFE stands for differential pressure feedback emissions and the sensor is the square metal box connected by hoses to the EGR valve. When you get a trouble code for the sensor you would need to test the sensor and the EGR valve.
for p0401: 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 culprit.
Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open. Clean out the EGR valve and/or tubing to remove deposits. Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model). Replace the DPFE sensor (with a good quality / OEM one). Replace the EGR valve.
the EGR valve is generally located on the backside of the intake plenum near the throttle body. There will be a metal pipe that goes in between the EGR valve and the exhaust manifold. If you follow that pipe you will run into to rubber looking hoses. the DPFE sensor will be on the other end of those two hose. The hoses are two different diameters as well as the fittings that they attach to. Make sure you have the right hose going to the right fitting.
What does that mean? EGR stands for Exhaust Gas Recirculation. It is part of the vehicle emmissions system, and is used to reduce combustion temperature and pressure to control Oxides of Nitrogen. There are generally three parts to the EGR system: the EGR valve, an actuator solenoid, and a differential pressure sensor EGR (DPFE). These things work together to deliver the correct amount of recirculation based on engine temperature, load, etc. The P0401 code means that OBD detected an insufficient amount of EGR.
Symptoms: You may notice drivability problems such as pinging (a.k.a. pre-ignition knock) when the engine is under load or the vehicle is at higher speeds. There may also be other symptoms.
Causes: A code P0401 most likely means one or more of the following has happened: * The DPFE (differential pressure feedback EGR) sensor is faulty and needs to be replaced * There is a blockage in the EGR (tube), most likely carbon buildup * The EGR valve is faulty * The EGR valve may not be opening due to a lack of vaccuum
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 culprit. * Use a vacuum pump and pull the EGR valve open while monitoring engine RPM's & DPFE voltage. There should be a noticable difference in RPM's with the EGR open * Clean out the EGR valve and/or tubing to remove deposits * Check the voltage at the DPFE, compare to specified values (refer to a repair manual for your specific model) * Replace the DPFE sensor (with a good quality / OEM one) * Replace the EGR valve
P0401 is: Exhaust Gas Recirculation Flow Insufficient Detected
The pipes are supposed to have pressure in them when the EGR valve is open, but sometimes, with the EGR closed may have vacuum... the DPFE compares the pressure/vacuum values from the 2 pipes (these 2 pipes take their pressure before and after an orifice) and knows how much recirculated exhaust gases go through the engine...
Your problem definitely sounds like an EGR problem, but it could be the EGR valve, the DPFE sensor, a clogged hose, a leak/clog somewhere in the EGR piping...
The code basically means your engine either is not getting the amount of burned exhaust gases it should back in the intake to lower NOx emissions or the DPFE sensor thinks your engine is not getting enough burned exhaust gases...