SPN 107 FMI 2: Meaning and Fix
This fault indicates the ECM detects erratic, intermittent, or incorrect data from the air filter 1 differential pressure sensor. Commonly seen after a forced DPF regeneration or engine washing, where moisture or soot temporarily disrupts the sensor signal. The ECM logs SPN 107 FMI 2 when the voltage deviates outside expected range for a defined period, often triggering a check engine light and reduced power.
Common Symptoms
- Intermittent Power Loss: Engine torque derate occurs sporadically, especially under high load or acceleration.
- Check Engine Light: Dashboard warning lamp illuminates and may flash intermittently during operation.
- Erratic Boost Pressure: Gauge fluctuates rapidly without corresponding throttle changes, confusing driver feedback.
- Harsh Idle: Engine idle becomes unstable when sensor signal glitches affect air-fuel ratio.
Probable Causes
- Sensor Wiring Chafing: Harness rubs against engine bracket causing intermittent short to ground or open circuit.
- Corroded Connector: Moisture ingress at sensor plug creates high resistance or signal dropouts.
- Damaged Sensor Diaphragm: Physical contamination or vibration cracks the internal pressure cell, causing erratic output.
- ECM Software Glitch: Firmware bug in older calibration versions misinterprets normal noise as erratic signal.
Advanced Technical Analysis
The ECM monitors the differential pressure sensor via a 5V reference, signal return, and signal wire. Using a 10 ms sampling rate, the microcontroller compares instantaneous voltage against a moving average. When three consecutive samples deviate by more than ±15% from the average, the ECM flags FMI 2. This debouncing filter prevents false triggers from normal engine vibration but can still fail if noise exceeds 200 mV peak-to-peak on the signal line.
Electrical analysis shows that a failing sensor often produces a sawtooth waveform at idle. The ECM’s internal analog-to-digital converter (10-bit resolution) interprets this as erratic data. Technicians using an oscilloscope on pin 2 of the sensor connector typically see voltage jumps from 0.8V to 3.2V within 100 ms. This pattern indicates either a loose ground connection or a failing internal voltage regulator inside the sensor.
Upon detecting an erratic signal, the ECM enters a safety fallback strategy: it freezes the last valid pressure value and uses a default airflow model, which reduces maximum torque by 25% and limits engine speed to 1800 rpm. This derate remains active until the next ignition cycle where the sensor signal must remain stable for 60 seconds. Technicians often observe this after engine washing when moisture temporarily shorts the sensor pins.
Long-term prevention requires inspecting the air filter housing for cracks that allow unfiltered air to contaminate the sensor port. In Deutz TCD 2013 engines, a known workshop scenario involves the sensor mounting bracket loosening over time, causing the diaphragm to resonate at certain RPMs. Replacing the sensor with an OEM part (Bosch 0 281 006 257) and applying dielectric grease to the connector solves most recurrent FMI 2 faults.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check sensor connector for bent pins, corrosion, and harness chafing near engine bracket.
- Oscilloscope Test: Monitor signal wire at idle; look for voltage spikes >200mV above baseline.
- Sensor Substitution: Swap with known good sensor and clear fault; if code returns, inspect wiring.
- ECM Calibration Check: Verify software version; update if older than 2019 release to fix debouncing logic.