SPN 131 FMI 13: Meaning and Fix
SPN 131 FMI 13 indicates the engine exhaust pressure sensor signal is out of calibration relative to the ECM’s expected atmospheric reference at key-on. This fault commonly appears after an ECM replacement or a forced DPF regeneration when the sensor offset was not relearned. Technicians frequently encounter this after swapping a DEF doser or performing an exhaust backpressure test without recalibrating the sensor via the diagnostic tool.
Common Symptoms
- Check Engine Lamp: Amber MIL illuminates immediately at key-on, often accompanied by a derate warning on the dash display.
- Reduced Engine Power: ECM activates torque limitation of up to 25% to prevent over-pressurization of the exhaust system.
- Erratic Exhaust Readings: Exhaust backpressure values on the scan tool deviate more than ±10 kPa from expected idle values.
- Failed Regen Cycles: DPF regeneration is inhibited because the ECM cannot verify correct exhaust pressure for injection timing.
Probable Causes
- Sensor Drift: The exhaust pressure sensor’s internal reference has shifted due to thermal aging or contamination from soot.
- ECM Replacement: New ECM lacks the learned offset for the specific sensor; calibration data was not transferred during programming.
- Wiring Harness Damage: Corroded pins or chafed wires in the 5V reference or signal circuit cause false calibration errors.
- Improper Sensor Installation: Sensor was replaced without performing the manufacturer-recommended atmospheric pressure relearn procedure.
Advanced Technical Analysis
The ECM monitors the exhaust pressure sensor at key-on, engine-off (KOEO) conditions, comparing the raw voltage to a stored atmospheric reference. If the difference exceeds ±3% (typically 0.15V on a 0.5–4.5V ratiometric sensor), the ECM sets SPN 131 FMI 13. This logic prevents the use of an uncalibrated sensor for EGR flow calculations and DPF regeneration control.
Electrically, the sensor receives a 5V supply from the ECM. A debouncing timer of 100 ms ensures the out-of-calibration condition is stable before the fault is logged. Intermittent opens or high resistance in the signal wire can mimic a calibration shift. Per Deutz factory manuals, the signal should read 0.50V ±0.02V at sea level KOEO.
Once the fault is active, the ECM defaults to a substitute value of 101.3 kPa (atmospheric) for exhaust pressure. This fallback disables the EGR differential pressure feedback, forcing the system into an open-loop control mode. Torque is derated linearly from 100% to 75% over 30 seconds to protect the turbocharger from overboost conditions.
Long-term prevention requires performing a sensor calibration reset after any ECM or sensor replacement. On MAN D26 engines, this is done via the diagnostic tool’s ‘Exhaust Pressure Sensor Relearn’ function while the engine is off. Technicians should also verify the sensor’s vent tube is unobstructed; a blocked vent causes internal pressure offset, triggering this fault repeatedly in cold climates.
Step-by-Step Troubleshooting Guide
- KOEO Voltage Check: Measure signal voltage at sensor connector; should be 0.50V ±0.02V at sea level with engine off.
- Perform Sensor Relearn: Use OEM diagnostic software to execute the exhaust pressure sensor calibration reset procedure.
- Inspect Vent Tube: Check the sensor’s atmospheric vent for blockages from ice, mud, or debris; clear if necessary.
- Verify Wiring Integrity: Check 5V reference and ground circuits for corrosion or intermittent shorts; repair as needed.