SPN 147 FMI 5: Meaning and Fix
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 ECM monitors a specific current range; when the signal drops below the threshold for more than 0.5 seconds, the fault is logged.
Common Symptoms
- No fuel economy data: Dash display shows dashes or zero for average fuel economy on gaseous engines.
- Check engine light active: Red or amber warning lamp illuminates on the instrument cluster immediately.
- Erratic sensor readings: Intermittent spikes to zero or full scale during steady-state operation.
- Engine torque derate: ECM reduces power to protect the fuel system when the fault is active.
Probable Causes
- Open sensor signal wire: Broken conductor between sensor pin A and ECM pin 33 on the 120-pin connector.
- Sensor internal failure: Open circuit inside the fuel economy sensor due to thermal stress or vibration fatigue.
- Corroded connector terminals: Oxidation on pins increases resistance, causing the ECM to detect an open circuit.
- ECM ground reference loss: Sensor ground wire open at ECM pin 12, preventing current flow through the circuit.
Advanced Technical Analysis
The ECM continuously monitors the fuel economy sensor via a 5V pull-up resistor to a 250-ohm sense resistor. Normal current is 4-20 mA. When current falls below 0.5 mA for 200 ms, the FMI 5 is set. The signal is sampled at 100 Hz and debounced with a 500 ms timer to prevent false triggers from electrical noise.
An open circuit is confirmed when the voltage at the ECM input pin rises to 4.8-5.0V (pull-up voltage) with no load. The diagnostic logic includes a short-circuit-to-battery check (FMI 3) and a range check (FMI 0/1). For FMI 5, the ECM expects a voltage > 4.5V at the sense resistor node.
Upon activation, the ECM sets the fuel economy parameter to a default value of 0.0 mpg and disables the fuel economy display. Torque is derated by 25% after 30 seconds of continuous fault. The derate is removed immediately when the fault becomes inactive. No limp-home mode is triggered.
Long-term prevention includes dielectric grease on connectors and routing the harness away from exhaust heat. Technicians often find this code after a DPF regeneration that overheated the sensor harness. In Deutz TCD engines, the sensor is located on the intake manifold; a common failure is chafing against the engine cover.
Step-by-Step Troubleshooting Guide
- Visual harness inspection: Check for cuts, burns, or chafing between the sensor and ECM, especially near exhaust components.
- Measure sensor resistance: Disconnect sensor; measure across signal and ground pins. Expect 250-350 ohms; open indicates failed sensor.
- Check ECM connector voltage: With key on, measure voltage at ECM pin 33 to ground. 4.8-5.0V confirms open circuit in harness.
- Perform wiggle test: While monitoring live data, wiggle the harness near known stress points to reproduce intermittent opens.