APPS is accelerator pedal position sensor (like a TPS on a gasser vehicle). more likely a failing or failed lift pump, but there is a mesh screen in the tank at the fuel pick-up point that can get plugged up with crap, especially if the truck sits a lot or you got some bad fuel. did you get a VP44 inlet fuel pressure yet?
Hope it did not go into the oil pan, sometimes it does not fall all the way down, try a very thin magnet. I put super glue on the key when I put in and put hear at 1:00 due to angle, its easier. Good luck!
Rick