P0401

P0401 Insufficient EGR Flow Detected

So you have changed the EGR valve and this p0401 code continues to pop up in the computer.  You are frustrated and ready to drive your car off of a cliff.  What does this P0401 diagnostic trouble code mean anyway?  The correct definition for this diagnostic trouble code P0401 is “Insufficient EGR Flow Detected“.  YES.. your EGR valve could be bad causing this code.  But chances are there is going to be something more causing your computer to flag this p0401 code.  In this article, we are going to explore a few different possibilities that can cause this code as well as testing strategies to be able to diagnose and fix this P0401 code once and for all.

EGR valve stands for Exhaust Gas recirculation.  The Exhaust gas recirculation valve takes exhaust gases from the exhaust manifold and puts them back into the intake chamber where they get sucked back into the engine and goes through the burning process for a second time.   Because the exhaust gases have a significant amount less oxygen then the atmospheric air coming into the engine, this will cool the burning process down resulting in lower emissions (NOX) coming out of the tail pipe.  The computer will flag your P0401 code in the computer when it recognizes that there is not enough exhaust gas going back through the intake chamber… or simply, low flow which causes the computer to flag the P0401 code.

How does your computer know how much exhaust gas is being burned if any? 

Different manufactures do it differently.  Some manufactures use the oxygen sensor to monitor the oxygen (air fuel) ratio in the exhaust as the engine is running.  The computer will watch the o2 sensor signal as it commands the EGR valve open.  If it does not see the right change in the o2 sensor signal, it will flag your P0401 code.  Other manufactures use the MAP sensor.  When the EGR valve is commanded open by the computer, the manifold vacuum will change and MAP sensor signal will change.  In addition, some manufactures (like Ford) use an EGR sensor or a DPFE sensor to tell the computer where the EGR valve is positioned or how much flow the EGR is currently flowing at any given time.  The EGR valve position sensor is located right on the EGR valve itself while the DPFE sensor (Digital Pressure Feedback EGR) is separate from the EGR valve.  It uses two hoses attached to the exhaust or exhaust manifold.  It senses the pressure difference (hence Digital Pressure Feedback) between the two hoses that are attached to exhaust and sends a signal back to the computer.  If either of these sensors go bad, they will give the computer an incorrect reading and the computer will “think” that there is incorrect exhaust gas flow and flag your P0401 low flow code.  YES.. these both type of sensors do go bad.  However, the DPFE sensor is a much more common failure component that the EGR valve position sensor.

How can I tell if my EGR valve is bad?

There are two types of EGR valves.  Both can go bad causing your P0401 code.  There are electronic and vacuum operated EGR valves.  Vacuum operated EGR valves are very easy to test.  Just hook up a vacuum pump to the EGR valve while engine is off and apply anywhere from 7 to 20 inches of vacuum and see if the EGR valve holds steady.  If it hold steady for 5 minutes, chances are it is good.  You need a hand held vacuum pump in order to do this.  A simple vacuum gauge will not do the trick.  Click here to see a reliable hand held vacuum pump that will be able to perform this test.

If you have an electronic EGR valve that you want to test because of a P0401 code in the computer, then your going to need to get your hands on a scan tool that has bi directional control.  That means you can not only view data, but also tell the on board computer on the vehicle to run certain tests through the scanner software.  From there, you can operate the EGR valve to see if it is working or not.  You may get upset because you have found out that you have an electronic EGR valve and you were hoping to test it using a hand held vacuum pump.  I got news for you, this is the 21st Centry.  Everything is going electronic these days.  I am surprised that they are still even using Vacuum operated EGR valves on these newer cars.  If you have a P0401 code in the computer and have an electronic EGR valve but do not have a scanner to test it, you can still remove it and check for carbon stuck in the pintial of the EGR valve and even if the pintial moves up and down like it should.

So my EGR valve checks out good but I still have this P0401 code?

A lot of times the computer flags this P0401 code because the intake ports intake either the throttle body or intake manifold are plugged up.  To see if this is the case causing your P0401 code, you can check to see if the EGR ports are plugged up by operating the EGR valve at idle and checking to see if the engine stalls out.  A good rule of thumb:  If the EGR ports are clean, then the engine should stall or run very rough when opening up the EGR valve when the engine is at idle.  If the engine idle is not affected when opening up the EGR valve, then either it simply did not open (bad EGR valve) or the intake or exhaust ports are plugged which would obviously cause your P0401 code.  If this is the case, the intake, throttle body or even exhaust ports will need to be cleaned out.  Sometimes this requires the removable of the throttle body and or the intake manifold.