SPN 931 FMI 1: Meaning and Fix
SPN 931 FMI 1 indicates the engine fuel supply pump actuator circuit reports a voltage or current below the normal operational range. This fault commonly appears after a forced DPF regeneration when the pump runs at maximum duty cycle and the ECM detects insufficient feedback. Technicians frequently encounter this after replacing the ECM without recalibrating the actuator offset.
Common Symptoms
- Hard Starting: Extended crank time due to insufficient fuel rail pressure during prime.
- Low Power: Engine torque derate above 1500 rpm as rail pressure cannot be maintained.
- Rough Idle: Erratic fuel delivery causes cylinder misfire at low engine speeds.
- No Start: Complete failure to start if actuator output falls below 5% duty cycle.
Probable Causes
- Open Circuit: Broken wire or corroded pin in the actuator supply or return line.
- Shorted to Ground: Actuator winding insulation failure causes low resistance and reduced current.
- Failed Actuator: Internal mechanical wear or coil degradation from thermal overstress.
- ECM Driver Fault: Internal low-side driver damaged by overcurrent or voltage spike.
Advanced Technical Analysis
The ECM continuously monitors the fuel supply pump actuator duty cycle command against actual current feedback using a precision shunt resistor. When the feedback current is below the calibrated threshold for 200 ms, the controller sets FMI 1. This indicates the actuator is not drawing enough current to maintain the commanded position, often due to increased resistance or an open winding.
Electrical analysis must include measurement of actuator resistance between 0.5 and 2.0 ohms per Bosch and MAN factory manuals. A debouncing timer of 50 ms prevents false triggering during transient loads. If resistance is within spec, the ECM driver output voltage at the connector must be verified under load; a drop below 10.5 V indicates a poor power supply or ground path.
Upon activation, the ECM enters a safety fallback mode by capping fuel supply actuator command to 40% duty cycle and limiting engine torque to 50%. This protects the high-pressure pump from cavitation. The fault is latched until ignition cycle, but the derate persists if the condition remains active. Real-world example: a Deutz TCD 2013 after DPF regeneration showed this code due to melted connector pins.
Long-term prevention requires checking actuator wiring for chafing near the engine block, a common issue on Mercedes-Benz OM471 engines. Technicians should always perform a actuator calibration routine after replacement using manufacturer software. Ignoring this code can lead to secondary damage of the high-pressure pump due to prolonged low lubrication flow from inadequate fuel supply.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check actuator connector for corrosion, bent pins, or melted insulation near exhaust.
- Resistance Check: Measure actuator coil resistance between 0.5-2.0 ohms; replace if open or shorted.
- Voltage Drop Test: Verify supply voltage at actuator connector under load; minimum 10.5 V DC.
- ECM Output Test: Command actuator 50% duty cycle with scan tool; confirm current draw above 1.5 A.