Ok First off I want to say I am pretty impressed on this forum I have learned alot. I am a big reader. But I am just pretty much puzzled at this point and before I spend more money I was wondering if anyone had any pointers for me. I have a 2001 Ford Explorer 2 door sport 4X4 The Explorer only has a bad startup when its cold. so If i do not run it for 12 hours, when I get in and turn it on it will be a bit rough, I have to keep my foot on the pedal otherwise it will stall out, after about 5-10 minutes it runs absolutely fine. ( For some reason on damp days it seems to start up a little more rough vs dry days.) But after its warm it will run completely fine. The spark plugs have been replaced along with new cords. The MAF has been replaced The EGR Valve has been replaced The IAC sensor has been replaced - What I am getting P0171 & P0174 - fuel trim or air error - something like that. Cylinder 4 is misfiring Oxygen sensor is bad just one I know where it is. I have seen on the internet that that a bad oxygen sensor will cause a cylinder misfire, but I am not sure if this is true or not. Anyone with any suggestions I would love to hear from. Thank you.