StangMuscle

Im hoping this post will help others who are having intermittent starting issues. With similar symptoms such as your car starting and dying, fuel pumps that occasionally do and do not prime, or make strange noises upon priming.
P0191= fuel rail pressure sensor, high/low voltage

Troubleshooting:
-Check battery terminals
-Check fuel rail pressure sensor (5v at harness)
-Check fuel rail pressure (35-40 psi)
-Check fuel pump fuse and relay (swap with another)
-Check voltage at fuel pump harness (my readings below, which are good)
-Test fuel pump (apply 12v from battery to pink/black, and green/purple wires. Ground black wire)
-Inspect fuel pump driver module, (fpdm) check voltages (my readings below, which should also be good)

Voltage at Fuel pump harness:
-Pink/black- 12.07v
-Green/Purple- 10.76v
-Light Blue/Yellow- 0.00-0.01v
-Black- Properly Grounded

Fpdm Harness Voltage:
-White- 12.07v
-Pink/Black- 5.83v
-Black 1- 5.65v
-Black 2- Properly Grounded
The two smaller wires were hard to get to given the ports is so small.
-White/red- 8.20v
-Blue/pink- 2.0v

I do not know where all these wires go to, but if you have different voltage readings hopefully this can help point you in the right direction. Other possibilities could be a short, the pcm is toast, or even a bad connection.

In my case the issue ended up being the fuel pump driver module. Our trunks are notorious for leaking, my module had water in the connections! I was able to use someone elses module to test if mine was toast. Sure enough it was, I plugged in the good fpdm and immediately the pump primed and the stang started perfectly again! Good luck! Please add any info you may have..