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I just spent a couple of days figuring my P0401 code out. I learned more than I wanted to know about this system. I have taken some photos and put together a slide show about how I troubleshot my system, this may help others. I am not a certified Auto Mechanic, nor do I work for Ford, etc. If I made a mistake, please let me know and I will correct. I was troubleshooting a 5.4 in my Expedition, but this may translate for a 4.6 too.