My 04 Explorer with a flex fuel 4.0 V-6 (120k) was dying, idling rough, emitting black smoke (extremely rich mixture) after it would warm up (about 6 miles) and of course a check engine light. The 1st code was P00900, ("Fuel Rail Pressure Switch). It needed to be replaced, the part is about $125.00 from the dealer. The thing is you have to remove the intake manifold to access it. Not a big deal, I am mechanically inclined and an average gear head, it took me 2 hours. You will need a Torx T-27 bit, I bought one for a 1/4" drive socket driver from Lowes. I used an 8 inch extension and was able to remove all 8 intake manifold bolts with it. The tools needed were flat screwdriver, T27 torx bit, extension and ratchet, needle nose pliers. Because your are removing the intake go ahead and buy some new gaskets (O ring style) from the dealer while your there. The intake is plastic and lifts off easily, thankfully, the eight intake manifolds bolts stay in place on the manifold while removing and installing so you don't have to thread them through the manifold and into the engine.
Follow the steps below and save yourself some money. BTW, make sure your remove the pressure from the fuel line before dis-assembly.