SPN 4375 FMI 2: Meaning and Fix
SPN 4375 FMI 2 indicates the ECM detects erratic, intermittent, or incorrect data from the aftertreatment 1 diesel exhaust fluid (DEF) pump drive percentage signal. This fault commonly appears after a forced DPF regeneration when the pump motor fluctuates unexpectedly, or after replacing the ECM without proper calibration. The commanded pump speed percentage deviates from expected values, compromising DEF dosing accuracy.
Common Symptoms
- DEF Dosing Interruption: System halts DEF injection, triggering derate within 30 minutes of operation.
- Check Engine Lamp Active: Red or amber warning lamp illuminates immediately upon fault detection.
- Torque Derate: Engine power reduces by up to 25% to protect aftertreatment components.
- Frequent Regeneration Cycles: Passive and active DPF regens occur more often due to incomplete NOx conversion.
Probable Causes
- Wiring Harness Chafing: Insulation damage near frame or turbocharger causes intermittent signal loss.
- DEF Pump Motor Failure: Internal brush wear or bearing seizure leads to erratic PWM feedback.
- ECM Calibration Mismatch: Software version incompatible with pump driver parameters after ECM swap.
- Corroded Connector Pins: Oxidation on J1939 or pump supply pins creates intermittent high resistance.
Advanced Technical Analysis
The ECM monitors the DEF pump drive percentage via a dedicated PWM input line sampled at 100 Hz. When the signal deviates more than ±5% from the commanded duty cycle for longer than 500 ms, the diagnostic sets FMI 2. The controller uses a debouncing timer to filter transient noise, but sustained erratic data triggers a fault.
Electrical analysis reveals that intermittent signals often stem from voltage drops below 10.5 V at the pump connector during high current draw. Using an oscilloscope, technicians can observe glitches or missing pulses on the PWM line. A 47 kΩ pull-up resistor inside the ECM can mask high-resistance faults, but not full opens.
Upon detecting erratic data, the ECM sets a torque derate of 12.5% immediately, escalating to 25% after 60 minutes of continuous operation. The DEF dosing valve closes, and the system defaults to a safe mode using only passive NOx reduction. Technicians frequently encounter this after replacing the ECM without performing the required pump calibration routine.
Long-term prevention includes dielectric grease on all DEF pump connectors and routing harnesses away from exhaust heat shields. In workshops, a common fix after a forced DPF regeneration is to verify the pump ground stud torque (12 Nm). A loose ground often mimics erratic data, especially on cold starts where resistance increases temporarily.
Step-by-Step Troubleshooting Guide
- Inspect Harness Routing: Check for chafing or pinching near frame rails and turbocharger heat shields.
- Measure Pump Supply Voltage: Verify 12.0-14.5 V at pump connector during key-on engine-off.
- Perform Pump Calibration: Use diagnostic tool to execute DEF pump prime and calibration routine.
- Check Ground Integrity: Torque pump ground stud to 12 Nm; measure resistance below 0.1 Ω.
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