SPN 4349 FMI 5: Meaning and Fix
SPN 4349 FMI 5 indicates the ECM detected current below normal or an open circuit in the circuit monitoring the Aftertreatment 1 SCR Dosing System Requested State. This signal uses four bits to report the system’s operational mode (e.g., dormant, dosing, protect). A low-current fault typically means the sensor or actuator line is broken. Technicians often encounter this after a harness chafes against the exhaust heat shield or when a dosing module connector corrodes, causing the system to default to ‘System error pending’ (0011b) and inhibit DEF injection.
Common Symptoms
- DEF Injection Stopped: SCR system halts dosing; ECM sets requested state to ‘System error pending’ (0011b) to prevent uncontrolled operation.
- Engine Power Derate: ECM reduces torque by up to 40% to protect aftertreatment, often triggering a gradual power loss on uphill grades.
- MIL and CEL Illuminated: Dashboard warning lamps activate immediately; fault code stored in non-volatile memory for 50 engine hours.
- Excessive Exhaust Smoke: Unburned hydrocarbons pass through due to inactive SCR, causing visible white or gray smoke during regeneration cycles.
Probable Causes
- Open Wiring Harness: Chafed or broken signal wire between ECM pin J1-42 and dosing module connector, common near exhaust bracket.
- Corroded Connector Terminal: Moisture ingress in 8-pin Deutsch connector at SCR dosing unit, increasing resistance above 10 ohms.
- Failed Dosing Module: Internal short or open circuit in the module’s state request feedback resistor network, drawing near-zero current.
- ECM Output Driver Damage: Overvoltage spike from short to battery+ fried internal MOSFET driver, causing open circuit on request line.
Advanced Technical Analysis
The ECM monitors the SCR requested state via a 5V pull-up resistor (1 kΩ) on the signal line. When the dosing module is connected, it pulls the line to a specific voltage level corresponding to the four-bit state code (e.g., 3.2V for ‘normal dosing’). An open circuit raises the voltage to 5V, while a short to ground pulls it below 0.5V. The ECM’s A/D converter samples at 100 Hz; if the voltage stays outside the valid range for 2 seconds, FMI 5 is set and the state defaults to ‘System error pending’ (0011b).
Electrical analysis shows that a broken wire or high-resistance joint (above 100 Ω) causes the current to drop below the 10 mA threshold. The ECM’s debouncing timer requires three consecutive 500 ms samples with current < 8 mA before logging the fault. Real-world cases on MAN D26 engines show this often occurs after a DPF ash cleaning service where the harness is accidentally pinched between the frame and the SCR canister.
When FMI 5 is active, the ECM forces the SCR system into ‘Protect mode against heat’ (0101b) if exhaust temperature exceeds 450°C, or ‘Shutoff’ (0111b) if below 200°C. This triggers a 25% torque derate after 10 minutes of continuous fault presence. The ECM also disables DEF pump operation and stores a freeze frame with exhaust temperature and engine speed at fault onset, essential for root cause analysis.
Long-term prevention involves dielectric grease on all SCR connectors and routing harnesses away from heat sources above 150°C. A common workshop scenario: after replacing the ECM on a Deutz TCD 7.8, the technician forgot to transfer the dosing module ground wire, causing this exact code. Using a breakout box (J1939 test tool) to measure signal voltage at the module while cycling the ignition key confirms the open circuit instantly.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check harness from ECM to dosing module for cuts, burns, or chafing near exhaust manifold and frame rails.
- Resistance Measurement: Disconnect both ends; measure signal wire resistance. Should be < 2 Ω. Open indicates break.
- Voltage Check at Module: Key-on, engine-off: measure voltage at dosing module pin 3. Should be > 4.5V. Low indicates open.
- ECM Output Test: Jumper signal pin to ground. If code changes to FMI 4, ECM driver is good; focus on module side.