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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.

P0087 is set when 1) rail pressure is above target rail pressure by more than 200~150at RPRV operating condition . This code is due to 1)fuel less than target value supplied to common rail or 2)excessive return of fuel supplied to common rail or 3)short to low voltage line in rail pressure sensor.