Fuel metering unit(MPROP) 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 operating current(with the method of duty-control) using RPCV signal, RPM and APS signal.

The lower Fuel metering unit current is, the more fuel is supplied to common rail. Thus it leads rail pressure to be high. On the contrary, the higher Fuel metering unit 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.

P0253 is set when "0"A 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 open of short to ground in control circuit or Fuel metering unit internal open.