SPN 4344 FMI 6: Meaning and Fix
SPN 4344 FMI 6 indicates the ECM detected current above normal or a grounded circuit on the Aftertreatment 1 Diesel Exhaust Fluid Line Heater 3. This fault commonly appears after a forced DPF regeneration when thermal stress cracks the heater element, causing a short to ground. The ECM monitors heater state (inactive, active, error, not available) and triggers this code when the sensed current exceeds the calibrated threshold, often due to a pinched wire or melted insulation in the harness near the DEF doser.
Common Symptoms
- DEF system inactive: Heater 3 fails to activate, preventing DEF thawing and causing dosing to stop below 0°C.
- Engine derate active: ECM reduces engine torque by up to 25% to protect the aftertreatment system from incomplete dosing.
- MIL and DTC lamp on: Dashboard warning lights illuminate immediately, often accompanied by a stop engine lamp in severe cases.
- Frequent regeneration fails: DPF regeneration cycles abort prematurely due to improper DEF dosing and exhaust temperature management.
Probable Causes
- Shorted heater element: Internal resistance of heater 3 drops below 0.5 ohms, creating a direct short to ground.
- Chafed harness wiring: Insulation wear on the heater supply wire at the frame rail or DEF line bracket causes ground contact.
- Corroded connector pins: Moisture ingress in the 2-pin connector near the SCR muffler leads to intermittent short circuits.
- ECM driver failure: Internal high-side driver of the ECM output stage fails shorted, permanently energizing the heater circuit.
Advanced Technical Analysis
The ECM applies a 12V PWM signal to heater 3 and reads current via a low-side shunt resistor. If the measured current exceeds 15A for more than 200ms, the firmware sets the ‘error’ state (10b) and latches FMI 6. This debouncing prevents false triggers from inductive spikes during engine cranking.
Electrical analysis shows that a grounded heater circuit creates a parallel path to chassis ground, bypassing the heater load. The ECM detects this as an abnormally low resistance (<0.5 ohms) during the pre-drive self-test. Actual factory data from Bosch EDC17 ECUs confirms a threshold of 12.5A ± 0.3A for fault activation.
Upon fault confirmation, the ECM sets the heater 3 state to ‘error’ and disables the DEF dosing below 5°C ambient temperature. A torque derate of 25% is applied after 30 minutes of continuous fault, and the SCR efficiency monitor is inhibited to prevent false NOx sensor codes.
Technicians frequently encounter this fault after replacing the ECM without transferring the heater calibration data. The new ECM may have a different current threshold, causing a false FMI 6. Always perform a ‘heater resistance learn’ procedure using the factory scan tool after ECM replacement to avoid repeat failures.
Step-by-Step Troubleshooting Guide
- Measure heater resistance: Disconnect harness at heater 3 and measure resistance between pins: should be 0.5–2.0 ohms.
- Inspect harness for shorts: With heater disconnected, measure resistance from heater supply pin to chassis ground: must be >1 M ohm.
- Check ECM output driver: Activate heater with scan tool and measure voltage at ECM connector: should pulse 12V only when commanded.
- Perform resistance learn: Use factory tool to run heater 3 calibration routine to adapt ECM current thresholds to actual heater resistance.