SPN 931 FMI 5: Meaning and Fix
SPN 931 FMI 5 indicates the Engine Control Module (ECM) has detected an open circuit or current below the normal operating range on the fuel supply pump actuator control circuit. This fault commonly appears after a forced DPF regeneration when the actuator harness is inadvertently damaged, or after replacing the ECM without proper calibration. The actuator regulates fuel flow to the high-pressure pump; an open circuit prevents this regulation, causing unstable rail pressure and potential engine stalling.
Common Symptoms
- Engine Stalling: Fuel supply actuator open circuit causes intermittent or immediate engine stall under load.
- Hard Starting: Extended cranking due to insufficient fuel rail pressure buildup from non-operating actuator.
- Low Power: ECM limits torque to protect the fuel system, resulting in noticeable power reduction.
- No Start: Complete failure to start if actuator circuit is fully open and ECM cannot command fuel flow.
Probable Causes
- Open Wiring: Harness chafing or pin breakage in the actuator circuit, common near engine vibration points.
- Connector Corrosion: Moisture ingress at the actuator or ECM connector, increasing resistance or causing open circuit.
- Actuator Failure: Internal coil winding breakage in the actuator solenoid due to thermal stress or age.
- ECM Fault: Damaged output driver stage within the ECM, often seen after a short circuit event.
Advanced Technical Analysis
The ECM microcontroller continuously monitors the current flow through the fuel supply pump actuator driver. When the measured current falls below a calibrated threshold (typically < 50 mA) for a debounce period of 500 ms, the diagnostic sets FMI 5. The ECM uses a high-side driver with integrated current sensing; an open circuit produces a zero-current reading that triggers the fault state immediately.
Electrically, an open circuit can present as a complete break in the harness, a corroded terminal with > 10 Ω resistance, or a failed actuator coil. The ECM’s diagnostic routine performs a pull-up test to detect open loads; if the voltage at the driver output does not drop when the driver is enabled, an open circuit is confirmed. Technicians should measure resistance between actuator pins (typically 2-5 Ω) and check for continuity to ECM pin.
Upon detecting FMI 5, the ECM activates a safety fallback by setting the fuel supply actuator to a default open position (if spring-return) or disabling it, causing the engine to rely on residual rail pressure. The ECM then initiates a torque derate of up to 40% and may illuminate the MIL. In severe cases, the engine will shut down after 30 seconds of continuous fault to prevent fuel system damage.
Long-term prevention requires inspecting the actuator harness routing during every DPF regeneration service. A real-world workshop example: a technician found that a loose cable tie caused the harness to rub against the engine block, creating an intermittent open that only appeared under vibration. Replacing the harness section and securing it with protective conduit resolved the fault permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check actuator connector and harness for damage, corrosion, or chafing near engine mounts.
- Resistance Check: Measure actuator coil resistance between pins; should be 2-5 Ω. Open indicates failed actuator.
- Continuity Test: Verify continuity from actuator connector to ECM pin; repair any open or high-resistance sections.
- ECM Output Test: With actuator disconnected, use a test lamp to verify ECM driver output; replace ECM if no pulse.