Full Diagnostic Guide — SPN 4375 FMI 6
1. What does SPN 4375 FMI 6 mean?
SPN 4375 identifies the Diesel Exhaust Fluid (DEF) pump motor current sensor or control circuit. FMI 6 indicates a current above normal or a grounded circuit. This means the ECM has detected excessive current draw, typically exceeding 5-7 amps during pump operation, signaling a short-to-ground or a mechanical overload condition in the pump motor assembly.
2. What are the most common symptoms when this code is active?
Common symptoms include a completely inoperative DEF system leading to SCR inefficiency, engine power derate (up to 25% torque reduction after 1 hour of operation), amber exhaust fluid warning lamp and MIL illumination, and audible grinding or clicking from the DEF pump area due to mechanical binding or electrical arcing in the motor windings.
3. How does the ECM determine that this specific failure (FMI 6) has occurred?
The ECM monitors the DEF pump motor current via an internal current sense resistor or Hall effect sensor. When the pump is commanded to run (PWM duty cycle 10-100%), the ECM expects a current between 0.5 and 4.5 amps. If the measured current exceeds 5.5 amps for more than 2 seconds continuously, the ECM sets SPN 4375 FMI 6, indicating a current above normal condition.
4. What is the difference between FMI 6 and other common FMIs for SPN 4375?
FMI 6 means current above normal or grounded circuit (excessive draw). FMI 5 indicates current below normal or open circuit (no draw). FMI 4 indicates voltage below normal or short to ground in the control circuit. FMI 3 indicates voltage above normal. FMI 6 specifically points to a short-to-ground or mechanical seizure causing high current, not an open or high resistance.
5. What are the most probable root causes?
Most probable causes are seized pump motor due to DEF crystallization (especially in cold weather), wiring harness damage causing a short-to-ground (chafed wires against chassis), corroded DEF pump connector pins creating a low-resistance path, contaminated DEF with non-spec urea concentration above 32.5% causing deposits, or a failed pump controller with welded relay contacts.
6. Can a purely mechanical issue cause this code without a faulty component?
Yes. A seized pump motor bearing due to DEF crystallization or frozen fluid in cold ambient temperatures below -11°C (12°F) can cause the rotor to bind, increasing mechanical load. This raises the current draw above the threshold even though the electrical components themselves are intact. This is common after extended idle in freezing conditions without proper DEF heater operation.
7. What default actions does the ECM take when this code is active?
The ECM immediately disables the DEF pump to prevent further damage and sets the amber warning lamp. After 30 minutes of engine operation with the fault active, the ECM initiates a progressive torque derate, reducing engine torque by 25% at 1 hour, 50% at 2 hours, and eventually limiting vehicle speed to 5 mph (8 km/h) until the fault is resolved and codes cleared.
8. How do I perform a basic functional test for this component?
With ignition on and engine off, command the DEF pump to prime using a diagnostic scan tool (typically 10-20 seconds). Listen for a smooth whirring sound from the pump. Use a digital clamp meter on the pump power wire; normal current should be 1.5-4.0 amps. If current exceeds 5.5 amps or the pump is silent with high current, suspect a seized motor or short circuit.
9. What specific electrical checks should I run before replacing parts?
Measure resistance between pump power pin and ground; should be >10 kOhms (not shorted). Check continuity of pump ground circuit (<0.5 ohms). Inspect connector pins for corrosion or bent terminals. Measure supply voltage at pump connector during prime command; should be battery voltage (12V or 24V depending on system). Use a megohmmeter to test insulation resistance (minimum 1 MOhm at 500V).
10. Is it possible that the ECM itself is responsible for this fault?
It is rare but possible. A failed PWM driver circuit inside the ECM can output a continuous full battery voltage instead of a modulated signal, causing the pump to run at full speed and draw excessive current. However, this is less common than wiring or pump issues. To verify, measure PWM duty cycle at the pump connector; if it stays at 100% regardless of commanded value, suspect ECM failure.
11. What is the complete step-by-step diagnostic procedure?
1. Record freeze frame data. 2. Visually inspect DEF pump harness for chafing, corrosion, or damage. 3. Disconnect pump connector; measure resistance between pump power pin and ground (should be >10 kOhms). 4. Reconnect and command pump prime; measure current with clamp meter. 5. If current >5.5A, remove pump and manually rotate impeller; if seized, replace pump. 6. If pump free, inspect wiring for short-to-ground. 7. Clear code and retest.
12. How can I prevent this fault from recurring?
Use only ISO 22241-1 compliant DEF (32.5% urea concentration). Ensure DEF tank heaters are functional before cold weather operation. Perform periodic DEF system maintenance, including draining and flushing the tank annually. Inspect pump connectors for corrosion every 500 hours and apply dielectric grease. Avoid extended engine idling in freezing temperatures without DEF heater operation.
13. Does this fault affect fuel economy, emissions, or engine lifespan?
Yes. The SCR system becomes inactive, causing NOx conversion efficiency to drop to near zero, increasing tailpipe NOx emissions by up to 90%. The resulting torque derate forces the engine to operate at lower RPM and higher load, reducing fuel economy by 5-15%. Prolonged derate can cause incomplete regeneration of DPF, leading to increased soot loading and potential engine damage.
14. Can I clear the code and continue operating the vehicle temporarily?
Clearing the code with a diagnostic tool will reset the derate timer, but the fault will likely reappear within minutes if the root cause persists. This is not recommended because it can lead to severe SCR catalyst damage and potential fines for emissions non-compliance. Only clear the code after performing repairs and verifying normal pump current draw (<4.5A).
15. When should I choose to replace the component versus repairing the wiring?
Replace the DEF pump assembly if current draw exceeds 5.5A and mechanical binding is confirmed by manually rotating the impeller (resistance greater than 2 N·m). Repair wiring if the pump passes the mechanical and current tests but harness resistance to ground is less than 10 kOhms or there is visible wire damage. Always replace the pump if it has been running with a short circuit for more than 10 seconds.
16. What type of diagnostic tool do I need to read this fault code?
You need a J1939-compatible diagnostic scan tool capable of reading SPN-FMI codes. Basic tools include handheld code readers with J1939 support (e.g., Nexas NX500, Autel MaxiCOM). For complete diagnostics, a professional tool like a JPRO Professional, Cummins INSITE, or Detroit DDDR is recommended to access live data, bi-directional controls, and freeze frame data specific to SPN 4375.
17. What can a professional J1939 scanner do that a basic reader cannot?
A professional J1939 scanner can command the DEF pump to run for functional testing, display live current and PWM duty cycle values for SPN 4375, read freeze frame data with ambient temperature and battery voltage at fault occurrence, perform DEF quality tests via refractometer data integration, and access OEM-specific diagnostic routines (e.g., pump priming, air purge cycles). Basic readers only show the fault code and generic description.
18. What are the key CAN bus parameters I should monitor when diagnosing this code?
Monitor PGN 65110 (DEF Pump Control) for commanded pump speed (0-100%), PGN 65112 (DEF Pump Current) for actual current in amps, and PGN 65114 (DEF Pump Voltage) for supply voltage. Also monitor PGN 65100 (DEF Level) and PGN 65102 (DEF Temperature) to assess if freezing is contributing. Normal current should be below 4.5A at full speed; any reading above 5.5A confirms FMI 6.
19. What is a PGN and how does it relate to SPN 4375?
A PGN (Parameter Group Number) is a 18-bit identifier in the J1939 protocol that groups related parameters for transmission over the CAN bus. SPN 4375 (DEF pump motor current) is transmitted within PGN 65112 (DEF Pump 2 Data). The PGN contains the SPN along with its data value and status bits. When the ECM detects FMI 6, it broadcasts the fault via DM1 (PGN 65226) containing SPN 4375 and FMI 6.
20. What components make up a complete J1939 Diagnostic Trouble Code (DTC)?
A complete J1939 DTC consists of four fields: SPN (Suspect Parameter Number) – 19 bits identifying the component/parameter (e.g., 4375 for DEF pump current); FMI (Failure Mode Identifier) – 5 bits describing the type of failure (e.g., 6 for current above normal); CM (Conversion Method) – 1 bit indicating how to interpret the SPN; and OC (Occurrence Count) – 7 bits counting how many times the fault has occurred since last clear.