SPN 931, labeled as the Engine Fuel Supply Pump Actuator, monitors and controls the electromechanical actuator responsible for regulating fuel delivery pressure and flow from the low-pressure supply pump to the high-pressure fuel injection system. This parameter is integral to diesel engine management across a broad range of heavy-duty applications, including Class 7 and Class 8 on-highway trucks, agricultural equipment, marine propulsion systems, and stationary power generation units. Engines from Cummins (ISX15, X15), Detroit Diesel (DD13, DD15, DD16), PACCAR MX-13, and Volvo D13 platforms commonly generate activity on this parameter. The actuator governed by this SPN directly influences fuel rail pressure, injection timing, and overall engine combustion efficiency — making it a high-priority diagnostic parameter whenever fuel system faults are present.
Technical Overview
The Engine Fuel Supply Pump Actuator is a solenoid-driven or stepper motor-driven control element integrated into the fuel supply (lift) pump assembly or the fuel metering unit located at the inlet of the high-pressure pump. In common-rail injection architectures — such as Bosch CP4.2, CP3.3, or Denso HP4 systems — the actuator takes the form of a fuel volume control valve (FVCV) or inlet metering valve (IMV) that modulates the quantity of fuel entering the high-pressure pump based on ECM command signals. The ECM outputs a pulse-width modulated (PWM) signal, typically operating at 50–1000 Hz with a duty cycle ranging from approximately 10% to 90%, to vary actuator position and thus control fuel supply pressure. Feedback to the ECM is achieved indirectly through the common rail pressure sensor (SPN 157) and fuel temperature sensors, allowing the engine control module to perform closed-loop regulation. In electronically controlled gear-type or vane-type transfer pumps found on Caterpillar C15 and C18 ACERT engines and John Deere PowerTech Plus series, the actuator may instead be a proportional pressure-relief solenoid mounted on the pump body itself. Normal commanded duty cycle values at idle typically range from 30–55%, rising to 70–85% under full-load demand, though these values vary by platform and fuel system architecture.
J1939 Network Behavior
SPN 931 is transmitted as part of the Electronic Engine Controller data broadcast on the SAE J1939 CAN bus, operating at the standard 250 kbps baud rate used throughout heavy-duty powertrain networks. While a fixed PGN assignment is not universally standardized across all OEM implementations for this specific actuator command value, it is commonly embedded within proprietary PGNs in the range of PGN 65,264 through PGN 65,279 for fuel system management data, or within OEM-specific peer-to-peer parameter groups transmitted from the Engine ECM (source address 0x00) to subsystem controllers. On Cummins CM2350 and CM2450 platforms, this data is broadcast at a 100 ms transmission rate during active engine operation. Detroit Diesel DDEC VI and later architectures transmit fuel actuator command data as part of their internal fuel management broadcast at 50 ms intervals. Aftertreatment control modules (ACM), transmission control units (TCU), and fleet telematics gateways may subscribe to this PGN to correlate fuel delivery performance with aftertreatment regeneration events and driveline load conditions. Diagnostic tools that connect via J1939 Data Link — including Cummins INSITE, Detroit DiagnosticLink, and Dearborn Group TMC RP1210-compliant adapters — can directly read the actuator commanded value and compare it against actual rail pressure response.
Diagnostic Importance
Faults associated with the Engine Fuel Supply Pump Actuator are treated as high-severity events by most ECM fault management strategies because fuel starvation or uncontrolled fuel pressure can cause immediate engine damage. When the ECM detects an out-of-range actuator response — such as rail pressure not achieving commanded values, actuator circuit open or shorted, or commanded duty cycle at maximum with insufficient pressure response — it typically activates a fuel system derate, reducing maximum fueling quantity by 25–50% to protect injectors, the high-pressure pump, and combustion hardware. On Volvo and Mack engines using the Volvo Electronic D-ECU, a sustained SPN 931 fault can trigger a three-stage engine protection shutdown. Caterpillar’s ACERT ECM logs this as a Level 2 or Level 3 event requiring immediate service attention before further operation. Ignoring active fault codes tied to this actuator risks catastrophic failure of the high-pressure CP4 pump due to cavitation caused by insufficient inlet pressure — a failure mode that, once it occurs, releases metallic debris throughout the entire fuel system, requiring injector replacement, fuel rail flushing, and complete pump replacement at significant cost.
Common Failure Patterns
Technicians most frequently encounter SPN 931 faults resulting from wiring harness degradation between the ECM and the actuator solenoid. Chafed insulation, corroded Deutsch or AMP connector pins, and broken wires near the fuel pump due to vibration are particularly common on high-mileage linehaul trucks. Internally, the actuator solenoid coil itself is prone to resistance drift — nominal resistance is typically 4–15 ohms depending on the application — and elevated coil resistance due to heat cycling causes weak magnetic actuation that manifests as erratic or insufficient fuel pressure response. Fuel contamination is another dominant failure driver; water intrusion or microbial growth in diesel fuel can cause the metering valve spool to stick or corrode, preventing the actuator from responding proportionally to ECM commands. On PACCAR MX-13 engines equipped with the Bosch CP4.2 high-pressure pump, low-lubricity diesel fuel accelerates internal pump wear that changes the effective flow gain of the inlet metering valve, causing the ECM to command abnormally high duty cycles that eventually trigger SPN 931 rationality faults. Calibration drift following fuel system component replacement without proper ECM re-initialization is also a documented cause on John Deere and Deutz TCD series engines.
Diagnostic Approach
Begin diagnosis by retrieving all active and stored fault codes using a J1939-compliant diagnostic tool — Cummins INSITE, Detroit DiagnosticLink, Caterpillar ET, or a universal RP1210 adapter with OEM software. Record freeze-frame data associated with the fault to understand operating conditions at fault activation. Perform a wiggle test on the wiring harness from the ECM actuator driver circuit to the pump-mounted connector while monitoring commanded and actual duty cycle values in real time. Measure solenoid coil resistance at the actuator connector and compare against OEM specifications — values outside the specified range confirm actuator failure. Verify supply voltage at the actuator connector under cranking and running conditions; a drop below 11.5V DC indicates a charging system or harness resistance issue. Use a lab-scope or graphing multimeter to observe PWM signal integrity at the actuator terminals, confirming clean square-wave output from the ECM driver. If circuit integrity is confirmed but rail pressure response remains abnormal, command actuator output directly through OEM service software to isolate ECM driver faults from mechanical actuator faults. Always perform a fuel system contamination check and filter restriction test before condemning the actuator, as restricted fuel supply is frequently misdiagnosed as actuator failure. Escalate to OEM dealer-level software when adaptation values or actuator trim calibrations require reset following component replacement.
Fault Codes for SPN 931
FMI 0: Data valid but above normal operational range (most severe)
SPN 931 FMI 0 indicates the engine fuel supply pump actuator is drawing current above normal operational range, triggering the most severe fault classification. This code frequently appears during cold weather startups when technicians notice extended cranking times or after fuel system component re
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FMI 1: Data valid but below normal operational range (most severe)
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
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FMI 2: Data erratic, intermittent or incorrect
SPN 931 FMI 2 indicates an issue with the engine fuel supply pump actuator, specifically data that’s erratic, intermittent, or incorrect. This fault commonly appears after electrical modifications or component replacements that affect the fuel delivery system. Technicians often encounter this fault
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FMI 3: Voltage above normal or shorted high
This fault indicates the ECM detected voltage above normal on the fuel supply pump actuator control circuit. The actuator regulates fuel pressure entering the high-pressure pump. In practice, this code often appears after a jump-start or battery disconnect, causing a voltage spike that damages the a
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FMI 4: Voltage below normal or shorted low
SPN 931 FMI 4 indicates the Engine Control Module detected voltage below normal or short-to-ground condition in the fuel supply pump actuator circuit. This fault commonly appears during cold startup sequences when pump inrush current causes voltage drops, or after fuel system maintenance when connec
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FMI 5: Current below normal or open circuit
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
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FMI 6: Current above normal or grounded circuit
SPN 931 FMI 6 is a critical fault code indicating an above-normal current or grounded circuit in the Engine Fuel Supply Pump Actuator. This fault frequently appears after ECM replacements or wiring harness repairs, where technicians may overlook grounding issues. Such situations lead to increased fu
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FMI 7: Mechanical system not responding properly
SPN 931 FMI 7 indicates the engine fuel supply pump actuator mechanical system is not responding properly to ECM commands. This fault commonly appears during high-demand operations when the fuel system cannot maintain adequate rail pressure. Technicians frequently encounter this code after fuel filt
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FMI 9: Abnormal update rate
This fault indicates the Engine Control Module (ECM) has not received a valid CAN message from the fuel supply pump actuator within the expected time window, typically 50-100 ms. Technicians often encounter this after replacing the ECM or performing a forced DPF regeneration, when the actuator contr
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FMI 11: Root cause not known
SPN 931 FMI 11 indicates an unknown root cause in the Engine Fuel Supply Pump Actuator. This fault often surfaces after ECM replacements or following fuel system maintenance. Technicians might notice this fault code when the vehicle experiences erratic fuel delivery, potentially leading to power los
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FMI 12: Bad intelligent device or component
SPN 931 FMI 12 indicates the engine fuel supply pump actuator has been identified as a faulty intelligent device by the ECM’s internal diagnostic algorithms. This fault commonly appears after ECM replacement when the new control unit detects inconsistent feedback signals from the actuator’s internal
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FMI 13: Out of calibration
SPN 931 FMI 13 indicates the Engine Fuel Supply Pump Actuator has lost its calibration reference. This fault commonly appears after an ECM replacement or a fuel pump swap where the actuator’s stroke range was not re-taught. Technicians frequently encounter this after forced DPF regenerations when th
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FMI 14: Special instructions
SPN 931 FMI 14 relates to special instructions concerning the Engine Fuel Supply Pump Actuator. This code frequently appears when there’s a mismatch between expected and actual fuel delivery parameters, often after an ECM replacement or reprogramming. Technicians may notice this fault during diagnos
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FMI 18: Data valid but below normal operating range (moderately severe)
The fuel supply pump actuator control signal operates below expected parameters, indicating insufficient pump command authority. This fault commonly appears during cold weather operations when fuel viscosity increases, requiring higher pump pressures. Technicians frequently encounter this code after
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FMI 31: Condition exists
SPN 931 FMI 31 indicates a condition with the engine fuel supply pump actuator. This fault often surfaces after ECM replacement due to mismatched control parameters. Technicians may notice irregular engine performance or fuel pressure issues. Common in scenarios where comprehensive ECM reprogramming