Hi Andy,
'P0191 fuel pressure sensor not a plausable signal' is a totally differant fault to 'P0191 faulty fuel rail pressure sensor'
The first one is I'm guessing the actual fault description you have and the second is your interpetation of the fault.
Remember the ECU can almost NEVER identify a faulty sensor as there is no DIRECT connection (ie always wiring in between and other parts of the system will affect how a sensor operates or reports its feedback) so a fault code alone cannot identify a faulty sensor.
The ECU is telling you that the output from the sensor which reports the actual fuel pressure is not PLAUSIBLE with what the fuel pressure should be reading under certain conditions.
So you may have an electrical fault with the sensor which would normally report as a sensor circuit fault or you may have a fuel pressure problem.
Hth Sean