Cummins ISB CM850 (2003-06) Fault Code: 559 (ISB) PID: 157 SPN: 157 FMI: 1 Fuel Pump Delivery Pressure Low – Data Valid but Below Normal Operational Range – Moderately Severe Level. The ECM has detected that fuel pressure is lower than commanded pressure.

Possible causes:

1)High inlet restriction
2)Excessive air in fuel
3)Loss of fuel prime during cranking
4)Plugged fuel filter
5)Low lift pump pressure
6)Stuck or restricted electronic fuel control actuator
7)System return (excessive return flow from the injector, high-pressure pump, or rail pressure relief valve)
8)Tampering.

If the drain line is restricted completely, the high pressure fuel pump will be damaged, causing internal leakage. Prior to pump damage, fault codes indicative of a drain line restriction will occur. Following the damage, these fault codes will go inactive and Fault Code 559, which is indicative of leakage, will be active. If troubleshooting Fault Code 559 results in replacement of the pump and there are inactive counts of Fault Codes 553 or 449, the fuel drain line must be inspected and the fuel drain line restriction measured. This is to reduce the possibility of repeat high pressure pump damage.

Possible repairs:

1)Add fuel to the fuel tank or perform repairs to the vehicle’s tank or fuel gauge.
2)Replace the electric fuel lift pump.
3)Replace the pressure-side filter and verify the pressure side fuel lines are not restricted.
4)Replace the high-pressure pump.
5)Replace the rail fuel pressure relief valve.
6)Tighten the injector high-pressure connector.
7)Replace the injector for the cylinder that is causing the high injector return flow.
8)Replace the high-pressure fuel pump.