p0191 is a fuel rail pressure sensor code, check this thread:
and from others thread:
"You may want to check out the Fuel Pump Driver Module (FPDM) itself, they're notorious for going bad. Our trucks don't have a return in our fuel system, so that module is responsible for adjusting the fuel pump to maintain the pressure. Easy access, just lower the spare tire, two bolts and you're done. "
" So, my 2004 F150 kept stalling on me and i couldnt figure out whatr was wrong with it so i started with the following..after replacing my fuel filter, then my fuel pump...estimated costed of a little over $550, i finally took it to a mechanic and the problem was the fuel pump module. so if ever you run into this problem check this first, its located above the spare tire and its a black box with an aluminum plate on the bottom that is bolted onto the steel frame, and since im up north salt and what not works its way in between these to metal and it corroides the aluminum plate and reveals the electronice circuit board which communicates with the fuel pump, onces this gets wet it shorts out and makes your truck stall. so if anyone runs into this problem, check out your fuel pump module. Hopes this saves some of you some money, unlike myself."