SPN 6802 FMI 6: Meaning and Fix
SPN 6802 FMI 6 reports that the aftertreatment 1 diesel exhaust fluid dosing system is frozen and a circuit fault (current above normal or grounded) is detected. This code often appears after a failed thaw cycle in cold climates, where ice in the DEF pump or lines causes a short-to-ground condition. Technicians encounter this frequently after extended idle periods in subzero temperatures, requiring careful inspection of the DEF heater circuit and pump wiring.
Common Symptoms
- Engine Derate: ECM reduces engine torque by up to 40% to protect the SCR system from DEF starvation.
- DEF Warning Lamp: Amber or red DEF warning lamp illuminates on the dashboard, indicating a frozen or faulty system.
- No DEF Injection: DEF pump fails to prime or inject fluid, causing increased NOx emissions and potential fault codes.
- Frozen Pump Noise: Audible clicking or humming from the DEF pump as it attempts to operate against ice blockage.
Probable Causes
- Shorted Heater Circuit: DEF tank or line heater wiring chafed against frame causing a direct ground and high current draw.
- Ice Blockage: DEF fluid frozen in pump or lines preventing flow and causing the pump motor to stall and short.
- Failed DEF Pump: Internal pump motor winding insulation breakdown leads to a current above normal condition.
- ECM Driver Fault: Internal ECM short circuit in the DEF pump or heater driver output, requiring ECM replacement.
Advanced Technical Analysis
The ECM monitors the DEF pump and heater circuits using a high-side driver with current sensing. When the measured current exceeds a calibrated threshold (typically 5A for the pump, 15A for heaters) for more than 500ms, FMI 6 is set. This logic prevents wire damage but can trigger falsely if ice increases mechanical load. Real-world data shows this code often follows failed thaw cycles where the heater duty cycle maxes out without melting ice.
Electrical analysis using an oscilloscope reveals that a true short circuit produces a flat current waveform at the supply voltage, while a frozen pump shows intermittent current spikes as the rotor tries to turn. The ECM’s debouncing timer (typically 2 seconds) filters out transient spikes, but sustained high current confirms a grounded circuit. Technicians should measure resistance between the pump terminals and chassis ground; values below 10 kΩ indicate a short.
When FMI 6 is active, the ECM immediately disables the DEF pump and heaters to prevent thermal damage. It then enters a safety fallback mode: torque derate to 60% of rated power, illumination of the DEF warning lamp, and logging of the event. The system will not attempt to thaw until the fault is cleared and the vehicle is restarted. This matches MAN and Mercedes-Benz factory protocols for SCR system protection.
Long-term prevention requires verifying the DEF heater ground path and using OEM-spec DEF fluid with proper freeze point (-11°C). In workshops, this fault commonly recurs after improper winterization—leaving non-insulated DEF lines exposed. A permanent fix involves installing heated DEF line kits and ensuring the ECM firmware is updated to handle extended thaw cycles. Deutz factory manuals recommend checking the pump motor brush wear as a root cause of intermittent shorts.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check DEF lines and pump for ice formation or physical damage. Look for chafed wires near the frame.
- Heater Circuit Test: Measure resistance of DEF tank and line heaters; should be 1-5 Ω. A short to ground reads <1 Ω.
- Pump Current Draw: Use a clamp meter on the DEF pump power wire. Normal draw is 2-3A; above 5A indicates a short.
- Thaw System Manually: Apply controlled heat (hot water, not open flame) to the DEF tank and lines, then clear the fault code.