SPN 155 FMI 5: Meaning and Fix
SPN 155 with FMI 5 indicates the engine speed/position sensor circuit current is below normal or an open circuit exists. The ECM detects a missing or weak signal from the magnetic pickup or Hall-effect sensor. In practice, this code commonly appears after a technician accidentally damages the sensor harness during a valve adjustment or after an engine overhaul. The ECM immediately flags the fault and may prevent engine start or limit RPM to a safe idle.
Common Symptoms
- No engine start: ECM disables fuel injection when missing engine position signal, preventing cranking or firing.
- Erratic tachometer: Tachometer drops to zero or jumps randomly as open circuit intermittently breaks signal path.
- Long cranking time: Engine cranks excessively before ECM detects a valid sync signal from the sensor.
- White smoke on crank: Unburned fuel exits exhaust due to incorrect injection timing from faulty position data.
Probable Causes
- Open sensor harness: Broken wire at sensor connector or chafed insulation near engine block or flywheel housing.
- Failed sensor coil: Internal winding open circuit in magnetic pickup sensor due to thermal stress or vibration fatigue.
- Corroded terminals: Oxidation on pins at ECM or sensor connector increases resistance, mimicking an open circuit.
- ECM pull-up failure: Internal ECM pull-up resistor open circuit prevents biasing the sensor line to proper voltage.
Advanced Technical Analysis
The ECM monitors the sensor circuit by applying a 5V pull-up voltage through a 10 kΩ resistor. When the circuit is intact, the signal line rests at approximately 4.8V. With an open circuit, the ECM reads a voltage near 5V (no current flow) and sets FMI 5. The ECM uses a debouncing timer of 200 ms to confirm the fault before logging it.
A complete open circuit yields zero current flow across the sensor. The ECM compares the measured current against a calibrated threshold, typically 2 mA. If current falls below this level for 100 ms, the fault is activated. Technicians should verify resistance between sensor signal and ground; an open circuit will show infinite resistance.
Upon detecting SPN 155 FMI 5, the ECM enters a safety fallback mode. It defaults to a fixed injection timing based on camshaft position alone, but if cam sensor data is also missing, the ECM disables injection entirely. A torque derate of 50% is common, limiting engine to 1200 RPM to prevent mechanical damage.
Long-term prevention includes inspecting sensor connectors for bent pins during every major service. Real-world example: a fleet truck repeatedly set this code after engine washes; water ingress corroded the sensor terminals. Replacing the sensor and applying dielectric grease resolved the issue permanently. Always check the ECM connector for pushed-back pins.
Step-by-Step Troubleshooting Guide
- Visual harness check: Inspect sensor-to-ECM harness for cuts, burns, or rubbing against engine components. Repair as needed.
- Measure sensor resistance: Disconnect sensor, measure resistance across its pins. Expect 500-1500 Ω; open circuit indicates failed sensor.
- Check ECM supply voltage: With sensor disconnected, measure voltage at harness side to ground. Should read 4.5-5.0V DC.
- Back-probe ECM connector: Verify continuity from ECM pin to sensor connector. Use a continuity tester; infinite resistance confirms open wire.