Cummins 559ISB Fault Code: Injector Metering Rail 1 Pressure - Data Valid But Below Normal Operating Range - Moderately Severe Level
Injector Metering Rail 1 Pressure - Data Valid But Below Normal Operating Range - Moderately Severe Level · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 559ISB (SPN 157, FMI 18) means the Cummins ISB CM2250 ECM detected that measured fuel rail pressure is at least 250 bar [3625 psi] below the commanded pressure for 7 seconds while the engine runs. It points to a fuel system that has lost the ability to build or hold rail pressure, and it can cause hard starting, low power, or a no-start.
High severity. This code disables DPF regeneration, closes the EGR valve, and will reduce engine torque if the engine keeps running with the fault active. It can also cause a no-start. It is not an immediate stop-the-engine emergency by itself, but it should be diagnosed before continued heavy-load operation.
What does Cummins error code 559ISB mean?
Fault code 559ISB (SPN 157, FMI 18) is set by the Cummins ISB CM2250 ECM when it detects that the measured injector metering rail (fuel rail) pressure is running below the commanded pressure by a significant margin. The ECM description labels this 'Data Valid But Below Normal Operating Range, Moderately Severe Level,' meaning the sensor signal itself looks valid, but the actual pressure reading is too low compared to what the ECM told the fuel system to produce.
The rail pressure control loop depends on the gear pump feeding the high-pressure pump, and the ECM adjusting a flow command to open or close the fuel pump actuator to add or reduce fuel supplied to the high-pressure pump. When rail pressure falls short of command, it usually means something upstream (low pressure fuel supply) or downstream (high pressure system leakage) is not doing its job.
The effect on the engine can be hard starting, low power, or in more severe cases, a complete no-start. Because the ECM also disables DPF regeneration and closes the EGR valve while this fault is active, and reduces torque with extended active running, letting it go unaddressed compounds both drivability and emissions system problems.
What triggers a Cummins 559ISB code?
The ECM sets fault code 559ISB when measured fuel rail pressure is below the commanded fuel rail pressure by at least 250 bar [3625 psi] for 7 seconds while the engine is running. This diagnostic runs continuously whenever the engine is operating. It is typical for the fault to go active under heavy engine load and become inactive again when the engine is lightly loaded, since the pressure shortfall is often most apparent under load.
Common causes of 559ISB
- Fuel inlet restrictions
- Fuel filter plugging
- Air in the fuel
- Fuel level below the suction line in the fuel tank
- Fuel gear pump output low
- Fuel gear pump pressure regulator sticking or leaking
- High-pressure pump capacity decay (high leakage to drain from the high-pressure pump)
- Fuel rail pressure relief valve leaking to drain
- Fuel rail pressure relief valve opening at too low a pressure (this condition is detected by fault code 3727)
- Injector fuel circuit leaking to drain
- External fuel leaks
- Pressure sensor reading more than 250 bar [3626 psi] lower than actual pressure
How to troubleshoot Cummins 559ISB: first checks
- Check for other, more precise fault codes first, especially electrical circuit faults. If fault codes 449, 1911, or 2311 are not present while 559ISB is active, check the ECM power supply for intermittent connections or battery issues, since intermittent ECM power can itself cause this code. Fault code 1117 may also be active alongside this condition.
- Verify vehicle system voltage using a diagnostic service tool's battery voltage data or a digital volt-ohm meter connected to the batteries. Refer to the OEM service manual for the specified voltage range.
- Isolate the low-pressure fuel system: confirm the system is properly primed, then check fuel gear pump pressure and inlet restriction. Note that accurate measurement of gear pump pressure and inlet restriction requires a special service tool that raises flow to rated levels, or operating the engine at full power.
- Check for air in the fuel. Bubbles indicate a leak allowing air entry, a severe inlet restriction causing cavitation, or a system that has not been primed. If restriction is not excessive, check suction fuel lines, OEM fuel lines, suction-side fuel filter assemblies, and tank standpipes.
- Check fuel level, making sure it is not below the suction line in the tank, and check for a plugged fuel filter.
- Isolate the high-pressure system for excessive leakage to drain, including the high-pressure pump, fuel rail pressure relief valve, and injector fuel circuits. Check high-pressure connector torque and tighten to specification if needed.
- If the fuel rail pressure sensor accuracy is in question, perform a key ON accuracy check only after the rail is fully depressurized. Wait at least 10 minutes after engine shutdown before checking, since high-pressure fuel takes time to bleed down to 0 bar [0 psi]. At key ON, a depressurized rail pressure sensor should read 0 ± 43 bar [624 psi].
- Verify ECM calibration is correct and check for calibration revision history covering this fault; recalibrate if necessary.
How the code clears
To validate a repair, start the engine and let it idle for 1 minute, or use a diagnostic road test / chassis dynamometer test with a mix of stop-and-go and steady-state or highway load operation, since some diagnostics require the engine to be loaded to run. The fault code status will change to INACTIVE as soon as the diagnostic runs and passes, and the amber CHECK ENGINE lamp will turn off at that time. On OBD-equipped engines, the malfunction indicator lamp (MIL) turns off after three consecutive trips where the diagnostic runs and passes. The Reset All Faults command in a diagnostic service tool can clear both active and inactive faults and extinguish the MIL on OBD applications. Never loosen fuel fittings while the engine is running, since the fuel pump, high-pressure lines, and rail carry very high-pressure fuel.
Affected models and serial ranges
559ISB appears in our records across 1 Cummins models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| CM2250 | Serial range not listed in source records |
Frequently asked questions
What does Cummins fault code 559ISB mean?
It means the ECM detected that measured fuel rail pressure is running well below the pressure it commanded, specifically at least 250 bar [3625 psi] low for 7 seconds while the engine is running. It signals that the fuel system, either the low-pressure supply side or the high-pressure side, has lost the ability to maintain proper rail pressure.
Can fault code 559ISB cause a no-start?
Yes. The effect listed for this code includes possibly hard starting, low power, or the engine possibly not starting at all. If the fault sets during cranking with no speed sensor codes present, it likely means the engine is not developing rail pressure during cranking, which can be caused by an unprimed fuel system, low gear pump pressure, or excessive leakage in the high-pressure fuel components.
Why does fault code 559ISB come and go instead of staying active?
It is typical for this fault to become active when the engine is under heavy load and go inactive again when the engine is lightly loaded, since the pressure shortfall often only becomes large enough to trigger the threshold under load. Intermittent causes such as high-pressure connectors that leak only when hot, floating debris causing intermittent fuel restriction, or a pressure relief valve opening prematurely can also make the fault hard to reproduce.
What should I check first if this code is active?
Start by ruling out other more precise fault codes, especially electrical ones, then check system voltage and ECM power supply connections. After that, focus on whether the low-pressure fuel supply system is functioning (properly primed, no restrictions, adequate gear pump output) before moving on to checking the high-pressure system for excessive internal leakage.
Is it safe to keep driving or working with fault code 559ISB active?
It is not described as an immediate emergency, but running the engine for an extended period with this fault active will cause the ECM to reduce engine torque, and it disables DPF regeneration and closes the EGR valve while active. It's best to get the fuel system diagnosed rather than continuing normal heavy-load operation.
How do I clear fault code 559ISB after a repair?
Validate the repair by idling the engine for 1 minute, or by running a diagnostic road test or chassis dynamometer test that includes both light and loaded operating conditions, since some diagnostics need engine load to run. Once the diagnostic runs and passes, the fault status changes to INACTIVE and the CHECK ENGINE lamp turns off; the MIL on OBD engines clears after three consecutive passing trips, or a Reset All Faults command can be used.
Could running out of fuel cause this code?
Yes. If fault code 559ISB is inactive and there are no ongoing performance complaints, it's possible the vehicle was run out of fuel, or the fault became active because the engine wasn't properly primed after a fuel filter replacement.