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.

P1186 is set when measured rail pressure, within the range where rail pressure is controlled by MPROP, is higher or lower than 200bar controlled as target rail pressure or when high pump amount is lowered than limited minimum value for lasting 2.0sec. Check whether less amount of fuel is delivered than target amount of fuel controlled by ECM, too much return amount delivered to common rail or stuck resulting from low voltage of rail pressure sensor.