Rail pressure regulator valve is installed in common rail and controls common rail pressure fast as regulating returning quantity of fuel which is delivered to common rail when fast rise of rail pressure is required like the moment of turning engine ON or when fast relief of pressure is required like at the point of deceleration.

The lower Rail pressure regulator valve current is, the more fuel is supplied to common rail. Thus it leads rail pressure to be high. On the contrary, the higher Rail pressure regulator valve current is, the less fuel is supplied to common rail.

P0091 is set when "0"A in control circuit of Rail pressure regulator valve(integrated with common rail) is detected for more than 0.11 sec.. This code is due to open of short to ground in control circuit or Rail pressure regulator valve internal open.