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As inputted rail pressure sensor signal, ECM of Commom rail diesel engine controls fuel metering unit(MPROP-intergrated with high press. pump) and rail pressure control valve(PCV-integrated with common rail) in order to maintain optimum rail pressure according to current engine rpm and load.

However when the problem that leads rail pressure to out of target value intended by ECM occurs due to mechanical or electronical reason, ECM limits engine performance and sets DTC by limiting fuel (stops injector operation) in order to prevent engine from being controlled abnormally. "rail pressure monitoring error" is the DTC which diagnose 1).supplying state of low pressure fuel and 2).mechanical operating conditions of high pressure pump and 3). RPCV indirectly based on RPS output voltage and RPS duty. thus, repair relavant to this DTC requires mechanics the total understand of fuel system.

Rail pressure is controlled by 1)only RPRV or 2)both RPRV and FPRV or 3)only RPRV depending on engine condition. These three modes are called ‘CPC mode’. During transition from one mode to another mode, if rail pressure is below the minimum rail pressure for each certain engine condition for more than 300ms. P1171 is set then, rail pressure is controlled by RPRV alone.