SPN 4375 FMI 3: Meaning and Fix
SPN 4375 FMI 3 indicates the ECM detected voltage above normal on the DEF pump drive command circuit. This fault often appears after a forced DPF regeneration when the pump runs continuously, or after replacing the ECM without recalibrating the pump driver. The ECM monitors the pump motor command signal; a short to battery voltage triggers this code, disabling pump operation and risking crystallization.
Common Symptoms
- Derated Engine: ECM reduces torque up to 40% to protect aftertreatment from untreated DEF injection.
- No DEF Dosing: Pump remains off; DEF injection stops, causing NOx conversion to fail completely.
- Lamp Activation: Red STOP lamp and amber WARNING lamp illuminate immediately on fault detection.
- Frozen Pump: Pump motor may lock due to crystallization if voltage spike occurred during cold soak.
Probable Causes
- Shorted Harness: DEF pump motor power wire chafed against chassis ground or battery positive rail.
- Failed Pump Motor: Internal winding short to housing or brush debris bridging the commutator to B+.
- ECM Driver Fault: PWM output stage failed shorted high due to overcurrent or thermal stress.
- Connector Corrosion: DEF fluid wicking into the pump connector creates conductive paths between pins.
Advanced Technical Analysis
The ECM microcontroller outputs a 0-100% duty cycle PWM signal to the pump motor driver. The driver feedback pin is pulled up to 5V reference. When the feedback exceeds 4.8V for more than 500 ms, the ECM sets FMI 3. This debouncing timer prevents false triggers from transient spikes during pump startup or regeneration events.
A short to battery voltage (12V or 24V) on the command line will immediately saturate the feedback ADC input. The ECM compares the measured voltage against a calibrated threshold table. If the signal stays above 4.8V for three consecutive 100 ms sampling windows, the fault is confirmed. Field data shows this most commonly occurs on chassis with aftermarket lighting wiring.
Once FMI 3 is active, the ECM stops sending PWM commands to the pump driver and sets the pump state to ‘inhibited’. The aftertreatment system enters a safe mode, disabling DEF injection entirely. The engine torque is derated by 25% initially, rising to 40% after 30 minutes of operation to prevent catalyst damage from untreated NOx.
Long-term prevention requires verifying the pump motor insulation resistance (>1 MΩ) and checking for DEF crystallization in the connector. Technicians frequently encounter this fault after replacing the ECM without performing the ‘DEF Pump Calibration’ routine. Always perform a pump prime test post-repair. A known Deutz workshop bulletin recommends replacing the pump harness as a pair with the motor.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check DEF pump harness for chafing near frame rails or exhaust heat shields.
- Voltage Measurement: With key on, measure pump command pin voltage; should be below 0.5V with pump off.
- Resistance Check: Disconnect pump; measure motor winding resistance (0.5-2.0 Ω). If open or shorted, replace pump.
- ECM Output Test: Use a breakout box to monitor PWM signal; if stuck high with pump disconnected, replace ECM.