SPN 270 FMI 31: Meaning and Fix
SPN 270 FMI 31 indicates the Engine Speed Sensor (Crankshaft) signal is continuously active outside expected parameters, often due to a short-to-power or sensor failure. This code commonly appears after a forced DPF regeneration when thermal stress damages the sensor wiring, or after replacing the ECM without recalibrating the sensor offset.
Common Symptoms
- Erratic Idle: Engine hunts or surges at idle due to unstable speed signal feedback to the ECM.
- No Start Condition: ECM fails to detect crankshaft rotation, preventing fuel injection enable.
- Torque Limitation: Engine power is reduced to 50% to protect drivetrain from erroneous speed data.
- MIL Illumination: Red malfunction indicator lamp lights up immediately after fault detection.
Probable Causes
- Sensor Shorted to Power: Internal short in sensor or harness to battery voltage causes constant high signal.
- Wiring Harness Damage: Chafed insulation against engine block or exhaust manifold creates intermittent contact.
- ECM Input Failure: Damaged ECM speed sensor input channel locks signal at a fixed high level.
- Incorrect Sensor Gap: Air gap >1.5 mm between sensor tip and reluctor ring reduces signal amplitude.
Advanced Technical Analysis
The ECM monitors the crankshaft speed sensor via a differential input circuit with a pull-up resistor to 5 V. Under normal operation, the signal toggles between 0.5 V and 4.5 V as the reluctor teeth pass. FMI 31 triggers when the ECM detects a continuous high voltage (>4.75 V) for more than 2 seconds without a low transition, indicating a condition exists that prevents proper signal cycling.
Electrical analysis reveals that a short-to-power on the signal wire (pin A) or a failed internal sensor transistor can lock the signal high. The ECM applies a debouncing timer of 500 ms to filter noise; if the high state persists beyond this, the fault sets. Technicians should measure voltage at the sensor connector with the harness disconnected—if >5 V is present, a short to battery is confirmed.
Upon activation of FMI 31, the ECM enters a safety fallback mode: it substitutes a default engine speed of 800 RPM and limits torque to 30% of rated. This prevents runaway conditions but forces the vehicle into limp-home mode. The ECM also disables cruise control and engine braking to avoid unintended acceleration from false speed data.
Long-term prevention requires inspecting sensor harness routing near exhaust components, as thermal degradation is a common root cause. In workshops, this fault frequently follows ECM replacement due to mismatched sensor calibration. Always perform a sensor air gap check (spec: 0.5–1.5 mm) and verify the reluctor ring for bent teeth, as seen after clutch or flywheel replacements.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor and harness for chafing, corrosion, or damage near exhaust manifold.
- Voltage Measurement: Measure signal voltage at ECM pin; if >5 V, repair short to battery.
- Sensor Resistance Test: Disconnect sensor; measure resistance between pins A and B (spec: 800-1200 ohms).
- Air Gap Verification: Insert feeler gauge between sensor and reluctor; adjust to 0.5-1.5 mm gap.