SPN 157 FMI 2Fuel SystemHigh
Injector Metering Rail 1 Pressure - Data Erratic, Intermittent or Incorrect
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Overview
Fault Code
SPN 157 / FMI 2
Category
Fuel System
Fault Type
sensor
FMI Category
erratic
Severity
High
Derate Risk
Yes
Regen Inhibit
No
Inducement Risk
No
Component Location: Fuel rail pressure sensor threaded directly into the high-pressure fuel rail - typically on the driver side of the engine near the fuel injectors.
Common Symptoms
- Fuel rail pressure reading erratic or jumping in live data
- Engine derate active - ECM cannot reliably control fueling
- Rough idle or misfires - injection timing and quantity affected by erratic pressure signal
- Intermittent fault - sets and clears with temperature or vibration changes
- Possible companion faults: SPN 157/3 (rail pressure sensor high), SPN 157/18 (rail pressure low)
- Hard start or no-start during fault active period
Root Causes
- Fuel rail pressure sensor connector - loose, corroded, or intermittent connection
- Fuel rail pressure sensor wiring harness - chafed or broken wire causing intermittent signal
- Fuel rail pressure sensor internal failure - sensor element intermittently failing
- Fuel rail pressure fluctuation - actual rail pressure unstable due to pump or regulator issue
- High-pressure fuel pump wear - pump output fluctuating causing erratic pressure
- Fuel pressure relief valve intermittently opening - causing pressure spikes and drops
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Derate Behavior
Power Derate
Yes
Mileage / Regen Notes
Sensor connector corrosion increases with age. HP pump wear causing actual pressure fluctuation increases at 500k+ miles.
System Interactions
SPN 157 FMI 2 interacts with the following systems and may trigger cascading faults if not resolved promptly.
Fuel rail pressure — injector timing and quantity affected
Engine power output — derate may be triggered to protect injectors
Aftertreatment regen — low fuel pressure can prevent regen completion
TSB Reference: Cummins TSB 4021756 - Fuel Rail Pressure Sensor Erratic Signal Diagnosis; Detroit TSB 17-008 - Fuel Rail Pressure Erratic Diagnosis
Common Diagnostic Mistakes
Replacing the sensor without verifying wiring continuity and supply voltage first.
Clearing codes without identifying and resolving the root cause — the fault will return.
Ignoring related active faults — cascading faults often share a root cause.
Skipping the connector inspection — corrosion and damaged pins are a leading cause of sensor faults.
Condemning the ECM before exhausting all wiring and component tests.
Replacing injectors without verifying fuel supply pressure — low supply pressure causes injector faults.
Ignoring fuel filter restriction — a clogged filter causes low rail pressure that mimics injector failure.
OEM Notes
Cummins
X15: Fuel rail pressure sensor connector corrosion is common - clean connector before replacing sensor. INSITE fuel system test required.
Detroit
DD15: Fuel pressure relief valve intermittent opening is a known issue on DD15 - causes erratic pressure. DDDL fuel system test required.
PACCAR
MX-13: HP pump wear causing pressure fluctuation is common at 500k+ miles. DAVIE4 fuel pressure analysis required.
Volvo
MP8: Fuel rail pressure sensor wiring harness routing near the exhaust is a common chafe point. PTT fuel system test required.
Related Faults in This Category
OEM System Resources
ASE Training Resources
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Related Diagnostic Articles
Deep-dive diagnostic page: SPN 157 FMI 2 — Full FSM Sequence & Repair Steps — includes wiring notes, sensor specs, and gated repair procedures.
Authority References
Diagnostic procedures on this page are informed by OEM service literature and industry standards.
Cummins QuickServe OnlineOfficial Cummins service manuals, fault code lookup, and TSB database.Detroit Diesel DiagnoCentralDetroit Diesel diagnostic procedures, DDDL documentation, and service literature.PACCAR MX Engine ServicePACCAR MX-11 and MX-13 engine service documentation and DAVIE diagnostic guides.Volvo/Mack Tech ToolVolvo and Mack engine fault code reference and PTT/Tech Tool diagnostic procedures.ASE — Diesel Truck Technician CertificationNational Institute for Automotive Service Excellence — T2 Diesel Engines certification standards.EPA Aftertreatment & Emissions GuidelinesU.S. EPA heavy-duty diesel aftertreatment compliance, DPF/SCR regulations, and NOx standards.
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