SPN 147 FMI 31: Meaning and Fix
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 the ECM without recalibrating the fuel economy parameters, leading to erroneous fuel consumption readings and potential derate.
Common Symptoms
- Erratic fuel gauge: Dashboard fuel economy display shows inconsistent or zero values during operation.
- Torque reduction: ECM may apply a 25% torque derate to protect aftertreatment systems.
- Check engine light: Red or amber MIL illuminated on dash with active fault code.
- Poor fuel tracking: Fleet management systems report implausible fuel consumption trends.
Probable Causes
- Sensor signal failure: Gaseous fuel flow sensor output stuck high or low due to contamination.
- ECM calibration error: Incorrect fuel economy parameters after ECM replacement or software update.
- Wiring harness fault: Open circuit or short to ground on J1939 CAN bus lines for fuel data.
- Virtual sensor misconfiguration: Fuel economy calculation not enabled or incorrectly mapped in ECU.
Advanced Technical Analysis
The ECM monitors the average fuel economy message transmitted via J1939 PGN 65196 (EEC2). If the received value remains outside the normal operating range (e.g., 0.0 to 100.0 km/kg) for more than 500 ms, the fault condition is set. Real-world, technicians see this after software updates where the fuel type parameter is accidentally set to diesel instead of gaseous.
Electrical analysis requires checking the CAN bus termination resistors (120 ohms) at the sensor node. A missing or incorrect termination causes signal reflection, leading to intermittent data loss. In practice, this fault often follows an engine harness repair where the CAN shield wire was left unconnected.
When the fault is active, the ECM replaces the missing fuel economy value with a default of 0.0 km/kg and sets a diagnostic trouble code. Torque derate is typically not immediate but activates after 10 minutes of continuous fault condition. This prevents false derates during transient fuel flow events like engine braking.
Long-term prevention involves verifying that the fuel flow sensor calibration matches the engine’s fuel type (CNG/LNG). After any ECM flash, perform a fuel economy learn cycle per manufacturer procedure. Workshop data shows this fault recurs if the sensor is not replaced within 500 hours after initial intermittent failure.
Step-by-Step Troubleshooting Guide
- Scan data review: Use diagnostic tool to read fuel economy value; if 0.0, suspect sensor or wiring.
- CAN bus inspection: Measure resistance between CAN H and CAN L; should be 60 ohms with power off.
- Sensor output test: With ignition on, measure voltage at sensor signal pin; should pulse between 0.5-4.5V.
- ECM parameter check: Verify fuel type parameter is set to gaseous and economy calculation is enabled.