Fuel Pressure Regulator Valve(FPRV) is integrated with high pressure pump and controls common rail pressure as regulating the quantity of fuel which is delivered to common rail. In order to control rail pressure to be optimum to current driving condition, ECM controls fuel pressure regulator valve operating current(with the method of duty-control) using RPCV signal, RPM and APS signal.

The lower Fuel 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 Fuel pressure regulator valve current is, the less fuel is supplied to common rail. Thus it leads rail pressure to be low. Therefore, when Fuel metering unit current becomes "0", as if there is open spot in Fuel metering unit circuit or connector is disconnected, maximum fuel quantity is supplied to common rail, so the pressure of common rail rise to maximum value.

P0252 is set when excessive current in control circuit of Fuel metering unit(integrated with high pressure pump) is detected for more than 0.22 sec.. This code is due to short to battery in control circuit or Fuel metering unit internal short.