SPN 4344 FMI 5: Meaning and Fix
SPN 4344 FMI 5 indicates the Aftertreatment 1 DEF Line Heater 3 circuit has detected current below normal or an open circuit. This fault often appears after a forced DPF regeneration when the heater element thermally fatigues and fractures internally. Technicians frequently encounter this on Mercedes-Benz OM471 engines after cold-weather operation, where the heater draws excessive inrush current before failing open.
Common Symptoms
- SCR Inefficiency: DEF dosing is inhibited due to inadequate line heating, causing NOx conversion to drop below threshold.
- MIL Illuminated: Red stop lamp and amber warning lamp activate on the dash, with a 30-second delay per J1939.
- Torque Derate: ECM commands a 25% torque reduction after 3 consecutive ignition cycles with the fault active.
- Frozen DEF Lines: In ambient temperatures below -11°C, DEF may freeze in the unheated line segment, blocking flow.
Probable Causes
- Open Heater Element: Heater 3 resistive element has fractured due to thermal cycling, causing infinite resistance.
- Harness Chafing: Wires to heater 3 chafe against the exhaust bracket near the SCR can, breaking the conductor.
- Connector Corrosion: Moisture ingress at the 2-pin Delphi connector causes pin oxidation and intermittent open circuit.
- Blown Fuse: 30A fuse in the auxiliary power distribution box opens due to repeated inrush current spikes.
Advanced Technical Analysis
The ECM monitors heater 3 current via a low-side driver with a 1 kHz PWM duty cycle. When the feedback voltage across the sense resistor remains above 4.5 V for 2 seconds, indicating less than 0.5 A, the ECM sets FMI 5. The debouncing timer prevents false triggers from inductive transients during pump activation.
An open circuit typically results from a wire break between the heater connector and the ECM pin A34. Resistance measurements should show 0.5–2.0 Ω across the heater at 20°C. Values above 10 Ω confirm an open element. The ECM also checks for voltage presence on the high-side supply before declaring the fault.
Upon detecting FMI 5, the ECM disables heater 3 and switches to a reduced duty cycle on heaters 1 and 2 to compensate. The aftertreatment controller then limits DEF dosing to 50% to prevent crystallization. A diagnostic trouble code is logged with a 1-hour soak timer before the lamp extinguishes.
For long-term prevention, technicians should inspect the heater harness routing to avoid contact with the exhaust manifold. In Deutz TCD 5.2 applications, adding a wire loom spacer reduces chafing. Real-world cases show that replacing the heater assembly and updating the ECM software (calibration revision 27.2) eliminates recurrent faults after DPF regeneration events.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check the heater 3 harness for chafing near the SCR bracket and confirm connector is fully seated.
- Resistance Test: Measure resistance between heater 3 pins; 0.5–2.0 Ω is normal. OL indicates open element.
- Voltage Drop: With key on, engine off, measure voltage at heater connector. Expect battery voltage ±0.5 V.
- Fuse Check: Locate and test the 30A DEF heater fuse in the auxiliary power box. Replace if open.
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