SPN 155 FMI 13: Meaning and Fix
SPN 155 FMI 13 indicates the engine position sensor has drifted beyond its calibrated range. This code commonly appears after a forced DPF regeneration when the sensor overheats or after replacing the ECM without performing a cam-crank correlation learn procedure. The ECM detects the signal timing deviation and sets the fault to protect engine synchronization.
Common Symptoms
- Hard start: Extended cranking time as ECM fails to synchronize cam and crank signals correctly.
- Rough idle: Irregular combustion events due to incorrect injection timing from misaligned sensor calibration.
- Reduced power: ECM activates torque derate strategy to prevent engine damage from timing errors.
- No start: Complete failure to start if sensor deviation exceeds ECM’s ability to infer position.
Probable Causes
- Sensor drift: Internal magnet or Hall element degradation from thermal cycling or vibration over time.
- ECM mismatch: Replacement ECM lacks stored calibration data for the specific engine sensor offset.
- Wiring resistance: Increased resistance in signal or ground circuits alters voltage thresholds read by ECM.
- Air gap error: Excessive physical gap between sensor tip and tone wheel due to wear or improper installation.
Advanced Technical Analysis
The ECM monitors the engine position sensor signal against a stored calibration curve derived during a learn procedure. When the measured pulse width or edge timing deviates by more than ±3% from the learned reference, FMI 13 sets. This check occurs every engine cycle, and the fault becomes active after three consecutive failed cycles.
Electrically, the sensor output is a digital square wave. The ECM measures the time between falling edges. A debouncing timer of 50 ms filters noise. If the deviation persists beyond this timer, the ECM latches the fault. Common root causes include connector pin fretting corrosion increasing contact resistance above 2 ohms.
Upon activation, the ECM enters a safety fallback mode: it freezes injection timing to a conservative default value and reduces maximum torque by 40%. The engine may still run but with poor efficiency. This protects against mechanical damage from pre-ignition or late burning caused by incorrect cam-crank synchronization.
Long-term strategy: After replacing the sensor or ECM, perform a forced calibration using a diagnostic tool like MAN Cats II or Bosch ESI[tronic]. In workshops, this fault often recurs if the technician skips the learn procedure. Always verify the tone wheel for missing teeth and measure sensor air gap with a feeler gauge (spec: 0.5–1.5 mm).
Step-by-Step Troubleshooting Guide
- Check air gap: Measure physical gap between sensor and tone wheel using a non-magnetic feeler gauge.
- Perform learn: Use factory scan tool to run engine position sensor calibration learn procedure.
- Inspect wiring: Measure resistance from sensor to ECM pins; repair if above 1.5 ohms per meter.
- Replace sensor: If calibration fails and wiring is sound, install OEM sensor and repeat learn procedure.