SPN 3556 FMI 31: Meaning and Fix
SPN 3556 FMI 31 indicates the ECM has detected a confirmed active fault condition in the Aftertreatment 1 Hydrocarbon Doser 1 circuit. This typically occurs when the doser valve fails to open or close correctly, often seen after a failed forced DPF regeneration attempt or when fuel dilution is detected in the exhaust stream. The ECM logs the condition and may disable further dosing events.
Common Symptoms
- DPF regen blocked: Active regeneration is inhibited; the system will not initiate a dosing event.
- Reduced engine torque: ECM triggers a torque derate of up to 25% to protect aftertreatment hardware.
- High exhaust temp: Unburned fuel in the DOC causes elevated exhaust temperatures above 650°C.
- DTC lamp illuminated: Amber warning lamp and stop engine lamp may activate after key cycle.
Probable Causes
- Doser nozzle clogged: Carbon buildup or crystallization blocks the fuel injection orifice.
- Wiring harness chafed: Short to ground or open circuit in the doser control wire near the frame rail.
- Doser driver failed: Internal ECM driver circuit for the doser solenoid is damaged or shorted.
- Fuel pressure low: Insufficient supply pressure from the dosing pump below 8 bar.
Advanced Technical Analysis
The ECM monitors the hydrocarbon doser driver circuit using a high-side switch with current feedback. When the commanded state does not match the sensed current (e.g., 0 mA when 1.2 A expected), the microcontroller sets a debouncing timer. After 2.5 seconds of continuous mismatch, the fault is confirmed as FMI 31. This logic prevents false triggering from transient noise.
Electrical analysis of the doser circuit shows a nominal coil resistance of 2.4 ohms at 20°C. The ECM applies a 12V pulse-width modulated signal at 100 Hz. A short to ground causes current above 5 A, while an open circuit yields 0 A. The debounce timer resets if the signal returns to normal for 100 ms, but a persistent error locks the fault active.
Once FMI 31 is set, the ECM executes a safety fallback: it disables the aftertreatment dosing system entirely, sets a torque derate of 25%, and illuminates both the amber warning lamp and the stop engine lamp. The system will not attempt regeneration until the fault is cleared via diagnostic tool. This prevents thermal runaway from unburned fuel.
Long-term prevention requires verifying doser flow rates during every major service. Technicians often find this code after replacing the ECM without transferring the doser calibration data. Real-world cases on MAN D26 engines show that a simple nozzle cleaning with ultrasonic bath resolves 60% of occurrences. Always perform a leak test after repair.
Step-by-Step Troubleshooting Guide
- Visual inspection: Check the doser harness for chafing near the exhaust manifold and frame rail.
- Measure resistance: Disconnect doser and measure pin-to-pin resistance; expect 2.2 to 2.6 ohms.
- Check supply voltage: With key on, verify 12V at the doser connector pin 1 relative to ground.
- Perform leak test: Activate doser via diagnostic tool; observe for proper spray pattern without drips.