SPN 6802 FMI 3: Meaning and Fix
This diagnostic trouble code indicates that the Aftertreatment 1 Diesel Exhaust Fluid Dosing System has detected a voltage above normal or a shorted high condition on the frozen sensor or heater circuit. In practice, this fault commonly appears after a vehicle is parked overnight in sub-zero temperatures and the DEF system fails to thaw during a pre-drive warm-up cycle. The ECM interprets the frozen state combined with an abnormal voltage reading as a hard fault, often preventing engine start or limiting torque until the system is serviced.
Common Symptoms
- Engine Torque Derate: ECM reduces engine power by up to 40% to protect SCR components from damage.
- DEF System Inoperative: Dosing pump remains idle; no DEF injection occurs, causing NOx conversion to stop.
- MIL and Amber Warning Lamp: Dashboard displays continuous amber warning and malfunction indicator lamp for SCR system.
- Frozen DEF Confirmation: Scan tool shows DEF tank temperature below -11°C with heater circuit voltage anomaly.
Probable Causes
- Shorted Heater Harness: Insulation breakdown in DEF line heater wires causes direct short to battery voltage.
- Failed DEF Tank Heater: Internal resistance drop below 2 ohms indicates a shorted heating element inside the tank.
- ECM Driver Failure: Output driver for DEF heater circuit fails closed, applying continuous high voltage.
- Corrupted Sensor Signal: DEF temperature sensor sends false voltage above 4.9 V due to internal short or moisture.
Advanced Technical Analysis
The ECM monitors the DEF temperature sensor and heater circuit continuously. When the sensor reports a temperature below -11°C, the ECM activates the heater. SPN 6802 FMI 3 triggers if the ECM detects a voltage above 4.8 V on the sensor signal line while the heater is commanded on, indicating a short to a 12 V or 24 V source. The diagnostic debouncing timer requires the condition to persist for 10 seconds before setting the fault.
Electrically, a short-to-high can occur when the heater power wire chafes against the sensor signal wire inside the DEF line harness. Bosch engineering manuals specify that the resistance between the heater supply and sensor signal should be >1 MΩ; any lower value indicates a short. The ECM uses a pull-up resistor to 5 V, and a short to battery voltage pulls the signal above the 4.9 V threshold, causing the FMI 3 code.
Upon activation of this fault, the ECM enters a safety fallback mode. It disables the DEF heater driver and sets a torque derate of 25-40% depending on OEM calibration. Deutz factory documentation notes that after 3 consecutive failed thaw cycles, the ECM locks the dosing system and requires a manual reset via diagnostic tool. The vehicle may still operate, but NOx emissions exceed legal limits.
Long-term prevention requires inspecting the DEF line harness for chafing at chassis contact points, especially near the frame rail. MAN service bulletins recommend using dielectric grease on all DEF heater connectors to prevent moisture ingress. A real-world example: a fleet truck in Minnesota repeatedly triggered this code after overnight parking; the root cause was a pinched heater wire near the DEF pump bracket, repaired by rerouting the harness and applying protective conduit.
Step-by-Step Troubleshooting Guide
- Visual Harness Inspection: Check DEF line harness from tank to doser for chafing, cuts, or melted insulation near hot surfaces.
- Heater Circuit Resistance Test: Measure resistance between heater supply and ground; should be 2-5 ohms. Short indicates failed heater.
- Sensor Signal Voltage Check: With ignition on, measure DEF temperature sensor signal voltage; must be below 4.9 V at -11°C.
- ECM Output Driver Test: Use breakout box to verify heater driver output; voltage should pulse width modulate, not stay steady high.