SPN 157 FMI 2 — Injector Metering Rail 1 Pressure – Erratic, Intermittent or Incorrect — Detroit DD15

Diagnostic reference for Detroit DD15 (2010–present) with SAE J1939 SPN/FMI reported by MCM/ACM/CPC. Focus on diagnosing the cause of erratic rail pressure…

Updated October 15, 2026

What it means

SPN 157 is Injector Metering Rail 1 Pressure. FMI 2 means erratic, intermittent or incorrect data. On Detroit DD15 the generic definition is that the fuel rail pressure reading is erratic or implausible.¹

Most likely causes

1. Rail pressure sensor or its wiring is degraded or intermittent — Erratic rail pressure data often traces to the rail pressure sensor or the harness/connectors feeding the MCM. Intermittent opens, chafing, corrosion or poor grounds can produce implausible readings.¹
2. Air in the low-side fuel supply or a supply restriction causing real pressure fluctuation — Low-side restrictions, failing lift pump, clogged filters or air ingress can cause real rail pressure fluctuation that the sensor reports as erratic. The platform notes real pressure fluctuation from air in the fuel or supply restriction as typical.¹
3. Electrical noise or reference voltage issue affecting the sensor signal — Noise on the 5 V reference or signal return, or a compromised sensor ground, can make the rail pressure signal appear erratic even when mechanical fueling is stable.¹

How to diagnose

1. Verify the code is active and review freeze-frame data for engine speed, load and rail pressure commanded vs actual to see if the erratic reading correlates with operating conditions.
2. Inspect the rail pressure sensor connector and wiring for chafing, moisture, corrosion and secure pin engagement; check harness routing near hot or moving components.
3. Check sensor supply voltage and ground integrity at the sensor connector with key on, and compare to spec; look for intermittent voltage drop during wiggle test of the harness.
4. Review fuel system maintenance history and inspect low-side components for signs of air ingress: filter seals, water separator, fuel lines and fittings; note any recent filter service.
5. Use diagnostic tool to monitor rail pressure PID at idle and under load; look for dropouts, spikes or implausible values that match the fault, and compare to commanded pressure.
6. If the electrical checks are inconclusive, perform a controlled road test while logging rail pressure, engine speed, load and fuel system PIDs to capture intermittent behavior.
7. Avoid direct high-pressure fuel system service for diagnosis; focus on confirming sensor/wiring integrity and fuel supply condition before any high-pressure component work.

Parts and tools you may need

• Diagnostic scan tool capable of Detroit MCM/ACM/CPC J1939 live data
• Digital multimeter
• Wiring diagram for DD15 fuel system
• Inspection light and mirror

Tools you’ll need

• diagnostic scan tool — Reads SPN 157 FMI 2 live data and freeze-frame to correlate erratic rail pressure with engine speed and load.
• digital multimeter — Checks rail pressure sensor supply voltage, ground integrity, and signal continuity during a wiggle test to find intermittent wiring faults.
• fuel pressure test kit — Monitors low-side supply pressure and detects air ingress or restriction that can cause real rail pressure fluctuation on the DD15.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $140–$200 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If rail pressure remains erratic after electrical and low-side checks, or if high-pressure fuel system testing is required, have a certified Detroit technician evaluate the vehicle. High-pressure fuel work involves safety risks and specialized tools.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources