2002 F150, P0401 Insufficient EGR Flow

This 2002 Ford F150 with a 5.4 liter engine came in with the service engine soon light on and a P0401 code stored. This same general test applies to most Ford, Lincoln and Mercury vehicles with EGR valves and DPFE sensors. Wire colors may vary and you may wish to consult a wiring diagram. It is easier to access the EGR valve and the DPFE sensor with the throttle body cover removed so I started by doing just that.

After I removed the cover I saw the problem. The two hoses that connect to the DPFE sensor were disconnected. I hooked the hoses back up and decided to do some testing to show how to test this system as I get many requests. Testing is as follows: The brown/white wire is the 5 volt reference voltage from the computer to the DPFE sensor. The grey/red wire is referred to as the signal return (it is a ground signal from the computer). The brown/light green wire is the actual DPFE sensor voltage sent to the computer from the sensor. All pinpoint testing must be done with a quality voltmeter. Do not use a test light on computer circuits unless specifically directed in repair information. With the key on the brown/white wire should have a nominal 5 volts. The grey/red wire should have near zero volts when checked from chassis ground to this wire. It should have near battery voltage when checked from battery positive to the grey/red wire. Be careful to not damage the wires.

The brown/light green wire should have between .5 and 1 volt when checked to chassis ground with the key on and engine off. The voltage should remain the same with the key on and the engine running and no vacuum applied to the EGR valve.

The voltage should increase as vacuum is applied to the EGR valve on a running engine. The follow picture shows that the vacuum applied to the EGR valve is about 22″ and the voltage is close to 3 volts. The engine should also start idling poorly and possible stall when vacuum is applied. The following are key points if you do not want to replace parts needlessly!!! If the engine idle does not change, the voltage remains low, the EGR valve holds vacuum and you can see or feel the pintle shaft move. DO NOT REPLACE THE EGR VALVE. It is not the problem. Instead the EGR ports to the throttle body are restricted and need to be cleaned. I hope to do a post on this soon, just waiting for one to come in. Now, if the idle does change as described above, the signal voltage does not and you have confirmed sensor reference and ground circuits are okay, then the DPFE sensor is faulty or there is a problem with the hoses as in my case with this truck. The only time the EGR valve should be replaced on these vehicles is if the valve will not hold vacuum or the pintle will not move when vacuum is applied or released. As of this writing I have never replaced an EGR valve on a Ford vehicle to correct and insufficient EGR Flow Code.

The following is just scan data that can easily be watched while test driving. One could extend the voltmeter leads to the inside of the vehicle and do the same test while some one drives the vehicle.

Be sure to follow proper safety procedures as testing requires the engine to be running. Mistakes can be tragic and fatal to both you and others.

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