As inputted rail pressure sensor signal, ECM of Commom rail diesel engine controls Fuel pressure regulator valve(intergrated with high press. pump) and rail pressure regulator valve(ntegrated 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 shuts engine down 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). rail pressure regulator valve 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 rail pressure is lower than target rail pressure by more than 200~150bar for more than 1.0 sec. in condition that rail press. is controlled by rail pressure control valve(PCV) or rail pressure is below the minimum limiting value(50~280bar). 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 fuel press. sensor.