The Engine Average Fuel Economy (Gaseous) parameter, identified by Suspect Parameter Number (SPN) 147, is a calculated value that monitors the average fuel efficiency of a gaseous-fueled internal combustion engine, typically using compressed natural gas (CNG), liquefied natural gas (LNG), or liquified petroleum gas (LPG). This SPN is critical for vehicle systems that rely on alternative fuels, including transit buses, refuse trucks, and heavy-duty vocational trucks equipped with engines from manufacturers such as Cummins (e.g., ISL G, Westport ISX12 G), Detroit Diesel (DD13/DD15 dedicated natural gas variants), and PACCAR (MX-13 NG). Unlike liquid-fueled engines, gaseous fuel systems require precise accounting for fuel mass flow, temperature, and pressure, making this parameter essential for fleet fuel management, emissions compliance, and driver performance monitoring. For diagnostic engineers, SPN 147 provides a direct window into the accuracy of the fuel metering system and the overall health of the engine’s air-fuel ratio control, which directly impacts combustion stability and engine longevity.
Technical Overview
SPN 147 is not derived from a single sensor but is a calculated value generated by the Engine Control Module (ECM) based on multiple input signals. The primary source data includes the gaseous fuel mass flow rate, typically measured by a thermal mass flow sensor or a Coriolis-type flow meter located in the high-pressure fuel line between the pressure regulator and the fuel injectors. Additionally, the ECM integrates engine speed (RPM), vehicle road speed from the transmission or ABS (via the J1939 bus), and total engine fuel consumed over a defined distance (e.g., miles or kilometers). The calculation algorithm is proprietary to each OEM, but fundamentally it uses a moving average window—often spanning 30 to 60 minutes of operation—to smooth transient events like idling or rapid acceleration. The signal type is a digital CAN message; the ECM internally computes the value in units of miles per gallon (MPG) or kilometers per liter (km/L), then broadcasts it as an unsigned 16-bit integer with a resolution of 1/512 (0.001953125) per bit, allowing a range from 0 to 125.5 units. The normal operating range for a modern gaseous heavy-duty engine under loaded highway conditions is typically 4.5 to 7.5 MPG (diesel gallon equivalent), with urban stop-and-go cycles yielding 2.5 to 4.0 MPG. Values outside these ranges, especially sudden drops or unrealistically high readings, indicate a fault in the fuel measurement system or a significant air-fuel ratio imbalance.
J1939 Network Behavior
On the SAE J1939 CAN bus, SPN 147 is transmitted within the Electronic Engine Controller 2 (EEC2) Parameter Group Number (PGN) 61444 (0xF004). This PGN is broadcast by the engine’s primary Electronic Control Unit (ECU), typically with a source address of 0 (Engine #1). The transmission rate is periodic, occurring every 100 milliseconds (10 Hz) when the engine is running, ensuring real-time data availability for other network nodes. The data is formatted as a 21-byte message, with SPN 147 occupying bytes 6 and 7 as a 16-bit unsigned integer. Other ECUs on the network—such as the Instrument Cluster, Body Controller, or Telematics Gateway—consume this data for driver displays, fuel economy tracking, and remote diagnostics. For example, a PACCAR MX-13 NG ECM will broadcast this value, which the dashboard ECU uses to update the instantaneous and average fuel economy gauges. In fleet management systems, the Telematics Gateway logs the average value over each trip segment, comparing it against historical data to detect fuel theft, driver inefficiency, or mechanical degradation. The J1939 network also allows for a “dummy” or default value of 0xFFFF (indicating “not available”) to be transmitted when the ECM detects a fault in the fuel flow sensor or the vehicle speed signal, preventing false data from corrupting downstream calculations.
Diagnostic Importance
Faults associated with SPN 147 are diagnostically critical because they directly implicate the accuracy of the fuel metering system, which is the foundation of combustion control in gaseous engines. When the ECM detects an implausible or out-of-range average fuel economy value—such as a sustained reading below 1.0 MPG or above 15.0 MPG—it will activate a Diagnostic Trouble Code (DTC) with SPN 147, typically paired with a Failure Mode Identifier (FMI) such as FMI 1 (Data Valid But Below Normal Operational Range) or FMI 2 (Data Erratic, Intermittent, or Incorrect). In response, the ECM may initiate engine protection strategies including derating of engine power (typically a 25% to 50% reduction in torque), limiting maximum engine speed to 1500 RPM, or forcing the engine into a “limp-home” mode that prioritizes safe operation over performance. For example, a Cummins ISL G engine will log a DTC for SPN 147 if the fuel mass flow sensor returns a signal that is inconsistent with the calculated fuel demand based on injector pulse width. Ignoring this fault can lead to severe consequences: continued operation with an inaccurate fuel economy calculation may mask a lean combustion condition, resulting in elevated exhaust gas temperatures that damage turbocharger bearings, exhaust valves, and the aftertreatment system. In extreme cases, sustained lean operation can cause pre-ignition or detonation, leading to catastrophic engine failure. Therefore, any active DTC for SPN 147 demands immediate diagnostic attention to prevent costly repairs and extended vehicle downtime.
Common Failure Patterns
Technicians encounter several recurring failure patterns with SPN 147 in the field. The most frequent is a faulty gaseous fuel mass flow sensor, which is a precision component subject to contamination from particulate matter (e.g., pipeline debris or compressor oil carryover in CNG systems). Over time, the sensor’s thermal elements become coated, causing a drift in the output signal that leads to a lower-than-expected fuel economy reading. A second common pattern is wiring harness degradation at the sensor connector, particularly on vehicles operating in harsh environments (e.g., refuse trucks with frequent vibration and moisture exposure). Corrosion on the sensor’s power or ground pins can cause intermittent signal loss, triggering an erratic data fault (FMI 2). Another failure mode is calibration drift in the ECM’s internal fuel map, often caused by software updates or improper aftermarket reprogramming. For instance, a Detroit Diesel DD13 NG engine may exhibit a consistent 10-15% error in SPN 147 after a flash update if the fuel injector trim codes are not properly set. Mechanical failures, such as a leaking fuel pressure regulator or a stuck-open fuel injector, can also cause the calculated fuel economy to deviate drastically from actual consumption. Finally, on vehicles with dual-fuel capability (e.g., diesel-gas blends), a failure in the diesel pilot injection system can cause the gaseous fuel flow sensor to report incorrect values, as the ECM’s algorithm assumes a fixed ratio between the two fuels. Each of these patterns requires a methodical approach to isolate the root cause rather than simply replacing the sensor.
Diagnostic Approach
When diagnosing a fault code involving SPN 147, the technician should follow a structured strategy beginning with a thorough scan of the J1939 network using a diagnostic tool such as Cummins INSITE™, Detroit Diesel Diagnostic Link (DDDL), or a generic J1939-capable scan tool. The first step is to verify the live data stream: observe the SPN 147 value during key-on, engine-off (should show 0 or “not available”), during idle, and under load. Compare this to the actual fuel dispensed over a known distance using a fuel card or fleet management report; a deviation greater than 10% warrants further investigation. Next, perform a circuit check on the fuel mass flow sensor: measure supply voltage (typically 5V or 12V from the ECM), ground continuity, and the signal output voltage or frequency (depending on sensor type). Reference values should be obtained from the OEM service manual—for example, a Cummins ISL G thermal mass flow sensor outputs a 0.5V to 4.5V analog signal proportional to flow. If the sensor signal is within specification, inspect the sensor’s inlet and outlet for debris or oil film; cleaning with a non-residue solvent may restore accuracy. If the sensor and wiring are intact, the next step is to check the vehicle speed source (SPN 84) and engine speed (SPN 190), as these are critical inputs to the fuel economy calculation. A faulty wheel speed sensor or a misconfigured tire size parameter in the ECM can cause a multiplicative error in SPN 147. Finally, if all circuit checks pass, escalate to OEM-specific software to perform a fuel flow sensor calibration or a fuel injector trim reset. For example, PACCAR’s service tool allows a “Fuel Metering Adaptation” procedure that recalibrates the ECM’s internal flow model. Only after exhausting these steps should the technician consider ECM replacement, as hardware failures in the controller are rare. This systematic approach ensures that the root cause is identified without unnecessary parts replacement, saving time and cost while restoring accurate fuel economy monitoring.
Fault Codes for SPN 147
FMI 0: Data valid but above normal operational range (most severe)
SPN 147 FMI 0 indicates the engine’s calculated average fuel economy for gaseous fuel exceeds normal operational parameters. This fault commonly appears in CNG transit buses after fuel system modifications or when fuel composition changes significantly. The ECM determines fuel economy based on injec
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FMI 1: Data valid but below normal operational range (most severe)
SPN 147 FMI 1 signals that the Engine Average Fuel Economy (Gaseous) value is below the normal operational range. This code often appears after a forced DPF regeneration on a natural gas engine, when the ECM detects implausibly low fuel consumption relative to engine load. The ECM continuously compa
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FMI 2: Data erratic, intermittent or incorrect
SPN 147 FMI 2 indicates the Engine Average Fuel Economy (Gaseous) signal is erratic, intermittent, or incorrect. This fault commonly appears after a forced DPF regeneration or when a technician replaces the ECM without recalibrating the fuel flow sensor. The ECM expects a stable signal from the fuel
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FMI 3: Voltage above normal or shorted high
SPN 147 FMI 3 signals a high voltage issue in the engine’s average fuel economy sensor, typically seen after ECM replacements or wiring modifications. This fault indicates that the sensor responsible for calculating fuel economy is receiving voltage levels beyond its normal operating range. Such sce
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FMI 4: Voltage below normal or shorted low
SPN 147 FMI 4 indicates voltage below normal threshold for the Engine Average Fuel Economy sensor in gaseous fuel systems. This fault commonly appears in natural gas or propane-powered vehicles after ECM replacement or wiring harness damage. The ECM detects insufficient voltage from fuel flow measur
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FMI 5: Current below normal or open circuit
SPN 147 FMI 5 signals that the Engine Average Fuel Economy sensor circuit for gaseous fuel is experiencing current below normal or an open circuit. This fault commonly appears after a technician replaces the ECM without recalibrating the fuel system or after rodent damage to the sensor harness. The
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FMI 6: Current above normal or grounded circuit
SPN 147 FMI 6 refers to a fault in the engine’s average fuel economy measurement circuit, often due to a grounding issue. This code is commonly encountered after technicians perform maintenance that involves the electrical harness, such as sensor replacements or ECM updates. A grounded circuit can l
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FMI 7: Mechanical system not responding properly
SPN 147 FMI 7 indicates the Engine Average Fuel Economy gaseous fuel system exhibits mechanical non-responsiveness, disrupting fuel delivery calculations and engine performance optimization. This fault commonly manifests during CNG or LPG fleet vehicle inspections when fuel pressure regulators fail
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FMI 9: Abnormal update rate
SPN 147 FMI 9 indicates the Engine Average Fuel Economy (Gaseous) parameter is not updating at the expected J1939 rate. This typically occurs after a battery disconnect or ECM reflash when the fuel economy calculation module fails to initialize. Technicians often see this code alongside a missing fu
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FMI 11: Root cause not known
SPN 147 FMI 11 indicates an anomaly in engine average fuel economy for gaseous engines, with the root cause being unknown. This fault often appears after engine control module (ECM) updates or during inconsistent fuel quality usage. Technicians frequently encounter this issue when there is a mismatc
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FMI 12: Bad intelligent device or component
This fault indicates a critical failure in the Engine Control Module’s fuel economy calculation circuitry for gaseous fuel systems. The ECM’s internal processing unit responsible for averaging fuel consumption data has malfunctioned. Technicians commonly encounter this code after CNG/LNG system conv
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FMI 13: Out of calibration
SPN 147 FMI 13 indicates the Engine Average Fuel Economy (gaseous) sensor or calculation is out of calibration. This code commonly appears after a forced DPF regeneration or after replacing the ECM, when the fuel economy offset values have not been rewritten. Technicians often encounter this fault w
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FMI 14: Special instructions
SPN 147 FMI 14 indicates a special instruction fault related to Engine Average Fuel Economy in gaseous engines. This code often appears after a vehicle undergoes extensive idle periods, leading to significant discrepancies in fuel consumption readings. Technicians frequently encounter this fault aft
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FMI 18: Data valid but below normal operating range (moderately severe)
SPN 147 FMI 18 indicates the Engine Control Module detects gaseous fuel economy values below manufacturer specifications. This fault commonly appears in natural gas transit buses during winter operations when prolonged idling reduces overall efficiency calculations. The ECM continuously monitors fue
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FMI 31: Condition exists
SPN 147 FMI 31 indicates the Engine Average Fuel Economy (Gaseous) signal has a condition exists fault, meaning the ECM detects an invalid or out-of-range data condition from the fuel flow sensor or virtual calculation. This code commonly appears after a forced DPF regeneration or after replacing th