SPN 3360 FMI 6: Meaning and Fix
SPN 3360 FMI 6 signals that the Aftertreatment 1 Diesel Exhaust Fluid Controller 1 has detected current above normal or a grounded circuit. This fault often appears after a forced DPF regeneration when thermal stress damages the DEF pump wiring insulation. The ECM monitors the dosing driver current; any reading exceeding the calibrated threshold triggers this code, potentially disabling SCR operation.
Common Symptoms
- Engine Derate: ECM reduces torque by up to 40% to protect the SCR system from over-dosing or under-dosing.
- MIL Illumination: Red or amber malfunction indicator lamp lights on the dashboard, often accompanied by a stop engine lamp.
- No DEF Dosing: Dosing valve remains closed; NOx conversion stops, leading to elevated tailpipe emissions.
- Frequent Regens: Passive and active regenerations occur more often due to soot buildup from incomplete combustion.
Probable Causes
- Shorted DEF Pump: Internal winding short in the DEF pump motor draws excessive current, tripping the driver circuit.
- Chafed Wiring Harness: Wires from ECM to DEF controller rub against the frame or heat shield, causing a ground fault.
- Failed DEF Heater: Heater element inside the dosing module shorts to ground, pulling current above the limit.
- ECM Driver Damage: Overvoltage or repeated overcurrent events burn out the internal MOSFET in the ECM output stage.
Advanced Technical Analysis
The ECM microcontroller uses a high-side driver with integrated current sense feedback. When the sensed current exceeds a factory-calibrated threshold (typically 4.5 A for a DEF pump) for more than 500 ms, the driver latches off and sets SPN 3360 FMI 6. The ECM then logs the event with a timestamp and freeze-frame data, including battery voltage and exhaust temperature.
Electrical analysis reveals that a debouncing timer prevents false triggers from transient spikes. If the overcurrent condition persists for three consecutive ignition cycles, the fault becomes active and initiates a torque derate. German manufacturers like MAN specify a maximum wiring resistance of 0.5 ohms in the dosing circuit; higher resistance indicates a partial short.
Upon activation, the ECM disables the DEF dosing driver and sets a fallback strategy that limits engine torque to 60% of rated power. The SCR catalyst efficiency drops below 50%, causing the ECM to command more frequent active regenerations. Bosch engineering documents note that the derate remains until the fault clears and a successful dosing self-test completes.
Long-term diagnosis should include measuring the DEF pump winding resistance (typically 0.8–1.2 ohms). Technicians frequently encounter this fault after replacing the ECM without updating the DEF controller firmware, causing driver mismatch. Preventive measures involve installing loom protectors near the DEF pump and verifying ground strap integrity on Deutz and Mercedes-Benz engines.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check the DEF pump harness for chafing, burns, or melted insulation near the exhaust manifold.
- Resistance Measurement: Disconnect the DEF controller and measure pin-to-pin resistance; a value below 0.5 ohms indicates a short.
- Current Draw Test: Using a clamp meter, verify the DEF pump current at key-on; over 5 A confirms a shorted pump.
- ECM Output Check: With the harness disconnected, measure ECM driver output voltage; if absent, the ECM may be faulty.