SPN 4349 FMI 2: Meaning and Fix
SPN 4349 FMI 2 indicates the Aftertreatment 1 SCR Dosing System Requests 1 PG transmits a state value that is erratic, intermittent, or incorrect. The ECM monitors the requested state (dormant, preparing, dosing, error, protect, shutoff) for consistency. This fault often appears after a forced DPF regeneration or when a dosing valve is replaced without proper ECM reset, causing the state to jump between normal dosing and system error pending.
Common Symptoms
- Intermittent Derate: Engine power intermittently drops, then recovers, as the ECM detects erratic SCR state transitions.
- Dosing Interruption: DEF dosing stops and restarts unpredictably, causing NOx conversion efficiency to fluctuate.
- Warning Lamp Flash: MIL and SCR lamp flash intermittently without a steady fault pattern, confusing the operator.
- State Cycling: Diagnostic tool shows SCR requested state cycling rapidly between dosing and error pending.
Probable Causes
- CAN Bus Noise: Electromagnetic interference on the CAN bus corrupts the state message from the ACM to the ECM.
- ECM Software Glitch: Firmware bug in the ECM causes incorrect state interpretation during wake-up or shutoff sequences.
- Faulty ACM: Aftertreatment Control Module internal fault sends erratic state bits on the J1939 network.
- Wiring Chafing: Chafed CAN_H or CAN_L wires near the exhaust system cause intermittent short circuits.
Advanced Technical Analysis
The ECM microcontroller monitors the 4-bit state field from PG 0xFEDC (Aftertreatment 1 SCR Dosing System Requests 1). An FMI 2 triggers when the received value does not match any valid state (0-7) or changes faster than the debouncing timer (typically 500 ms). This indicates a physical layer or logic error in the ACM-to-ECM communication.
Electrical analysis reveals that a single corrupted bit can shift the state from ‘normal dosing’ (0010b) to ‘system error pending’ (0011b), causing false alarms. The ECM’s input capture unit uses a 3-sample majority filter; if two out of three samples disagree, the FMI 2 is set. This commonly occurs when CAN bus termination resistors are out of spec (60 ohms ± 10%).
Upon detecting erratic state data, the ECM enters a safety fallback: it freezes the last valid state for 2 seconds, then forces the system into ‘protect mode against heat’ (0101b) to prevent DEF crystallization. A torque derate of 25% is applied until the state stabilizes. This protects the SCR catalyst but reduces vehicle productivity.
Long-term diagnosis requires a CAN bus oscilloscope capture during the fault. Real-world workshops report this code after aftertreatment module replacement without performing a ‘J1939 network configuration’ reset. Prevention includes verifying all CAN connections are twisted and shielded, and updating ACM firmware to the latest MAN or Deutz release that improves state debouncing.
Step-by-Step Troubleshooting Guide
- Capture CAN Log: Use a J1939 analyzer to log PG 0xFEDC for 10 minutes; look for invalid state transitions.
- Check Termination: Measure CAN_H to CAN_L resistance at the diagnostic port; must be 60 ohms with power off.
- Inspect ACM Wiring: Visually check ACM connector pins for corrosion or bent terminals; repair any damage found.
- Flash Latest Firmware: Update both ECM and ACM to latest factory calibration; clear fault and test drive for 30 minutes.