Code P0401 I recently got a code 401 on 2000 explorer 5.0. Thanks to some friendly posts I was able to troubleshoot and avoided pulling the EGR valve. I just wanted to share the troubleshooting technique I did. There are 3 working/moving parts, forgive me if I do not know their correct names. 1. EGR valve - I pulled the vacuum hose, started the engine and then pulled a slight vacuum on EGR valve. Engine speed slowed signficantly and then stalled. This tells me the EGR valve was doing okay. Also, with the engine off, I was able to pull vacuum on the EGR valve and it held, so no diaphragm leaks. 2. EGR position regulator. This is controllled by the PCM and depending on the voltage it gets, it controls the amount of vacuum to the EGR valve. - I pulled the sensor, connected it to 12 volt source at the terminals (search for which one is positve, there are pictures out there) with vacuum connected to the engine vacuum port. With no voltage, slight vacuum pulled to atmosphere, and none to the EGR connection port. With 12 volts, vacuum pulled to EGR connection port. So this seems okay. Also, there is info out there on the terminal to terminal resistances, I do not have this handy right now, but I like the function test, that let me know it was working. 3. DPFE sensor - This senses the differential pressure across an orifice in the exhaust gas return line from exhaust manifold back into the intake. This was the only thing that I did not test, since the first two components were okay, I thought this was the culprit. There are some tests out there that require the engine running at temp but for me that seemed a bit of trouble, I really did not want to get on top of the hot engine and try to read voltages, the DPFE sensor is in the back of the intake. So, I pulled the DPF sensor and replacement ones were ~ $ 50 and I had read they may or may not have much of a service life. I just decided to go to the local Pull - A - Part and for $6, I found a DPFE sensor that matched mine. I put it in the code went away. I am glad I did not have to pull the EGR valve, having done this before on another engine, I was not looking forward to that, and this one is not the easiest of access. Maybe someone who has this code can benefit from this post. I find this site so useful, I just want to try to contribute to help others as I have been helped.