Code Details
| Display code | SPN 157 FMI 18 |
|---|---|
| SPN | 157 |
| FMI | 18 |
| OEM code | None listed |
| Manufacturer | Cummins |
| System | Engine �?fuel system |
| Component | High-pressure fuel rail / common rail system |
| Source address | Unknown or not applicable |
| Severity | medium |
| Review status | source backed |
| Source confidence | medium |
| Last reviewed | 2026-06-11 |
Plain-English Meaning
The ISB 6.7 uses a Bosch common rail injection system that requires very high fuel pressure for proper combustion. When the rail pressure falls below the ECM's commanded target, the engine cannot inject fuel at the correct quantity and timing, resulting in power loss, rough running, and smoke. This is a relatively common fault in higher-mileage ISB 6.7 engines.
SPN 157 is Injector Metering Rail 1 Pressure. FMI 18 indicates a valid reading below the normal operating range. On the Cummins ISB 6.7 (CM2150/CM2350 variants), fuel pressure is monitored by a rail pressure sensor. The ECM compares commanded vs. actual pressure and adjusts metering valve position. A persistent below-normal reading despite maximum metering valve opening indicates pump wear, injector leak-off, or supply side restriction. Cummins Insite provides actual vs. commanded pressure and a cylinder contribution test to isolate fuel system faults.
Common Symptoms
- Loss of power under load
- Hard starting or extended cranking time
- Black or gray smoke under acceleration
- Rough idle or engine stumble
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- High-pressure pump worn (CP3 or CP4 depending on model year)
- Leaking fuel injectors bleeding rail pressure between injection events
- Restricted primary or secondary fuel filter
- Lift pump / supply pump delivering inadequate pressure or flow
- Damaged fuel rail pressure sensor providing an incorrect reading
First Checks
- Connect Cummins Insite and compare actual rail pressure to commanded pressure at idle and under load
- Replace primary and secondary fuel filters as a first diagnostic step
- Test supply pump output volume and pressure to verify adequate feed to the high-pressure pump
- Use Insite cylinder contribution test to identify any injectors with significantly higher leak-off than others
- If a CP4 pump is fitted (2013+ model years), check for metal contamination in the fuel system �?CP4 pump failure can contaminate the entire system
Can I Keep Driving?
Power loss will be noticeable, especially under load. If injector leak-off is the cause, continued operation without repair can lead to increased injector damage. Diagnosis should be prioritized.
Related Codes
Related Lookup Pages
Sources
- Cummins INSITE Service Tool — Public Reference Documentation Cummins Inc. · oem · accessed 2026-06-11 · confidence medium
Source: Cummins Inc., Cummins INSITE Service Tool — Public Reference Documentation. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - QuickServe Online Cummins Inc. · oem · accessed 2026-05-05 · confidence medium
Source: Cummins Inc., QuickServe Online. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Does the Cummins ISB 6.7 use a CP3 or CP4 fuel pump?
Earlier ISB 6.7 applications used a Bosch CP3 high-pressure pump, which is known for relatively good durability. 2013 and later CM2350 variants may use a CP4-style pump. The CP4 is more sensitive to fuel contamination and lubricity. If the pump is a CP4 and fails catastrophically, the debris it generates can contaminate injectors, fuel lines, and the rail �?requiring a more extensive repair.
Can I drive a Cummins ISB 6.7 medium-duty truck to a shop with SPN 157 FMI 18 active?
If the power loss is manageable and no metal contamination is suspected, a short drive to the nearest shop is generally acceptable. If the truck is stumbling severely or if CP4 catastrophic failure is suspected (unusual noise, sudden complete power loss), stop operation and investigate before continuing to prevent further fuel system contamination.
How does the ISB 6.7 fuel system compare to the Duramax or Power Stroke for fault diagnosis?
The ISB 6.7 uses a Bosch common rail system similar in principle to the GM Duramax and Ford 6.7 Power Stroke. All use a high-pressure pump, common rail, and electronically controlled injectors. The primary differences are in the specific pump model (CP3/CP4), injector style (Bosch solenoid vs. Denso piezo vs. Bosch piezo depending on application), and the OEM diagnostic tool used for pressure data and cylinder testing.