"P0191 Fuel Rail Pressure Sensor Circuit Range/Performance"

Car fumbles around in idle when it's cold out, nearly stalling. What are the causes for this light?

Side note: heading out to my shop to put in brand new Bilstein HDs and some wheel bearings... stoked for these!

 

There is a vacuum hose that goes to a sensor that comes from the regulator. If that hose comes off it will lean it out or make it run weird. Maybe the sensor is going bad.

 

Strange as the light has only come on once. It still seems to fluctuate in idle as the car warms up but no further codes are set.