SPN 3361 FMI 2: Meaning and Fix
This fault indicates the ECM has detected erratic, intermittent, or incorrect data from the Aftertreatment 1 Diesel Exhaust Fluid Dosing Unit 1. This typically occurs when the dosing unit’s internal pressure sensor or solenoid feedback circuit produces unstable voltage readings. Technicians frequently encounter this code after a forced DPF regeneration that was interrupted, or after replacing the ECM without recalibrating the dosing unit. The ECM compares expected dosing rates against actual feedback; any deviation beyond 15% for more than 3 seconds triggers this FMI 2.
Common Symptoms
- Intermittent derate: Engine power reduces sporadically, especially during low-load operation, as the ECM limits torque to protect the aftertreatment system.
- Malfunction lamp on: The amber warning lamp illuminates, often accompanied by a check-engine light that may flash intermittently.
- Regeneration disabled: Active DPF regeneration is inhibited by the ECM until the dosing unit signal stabilizes or the fault is cleared.
- Poor DEF consumption: DEF dosing becomes erratic, causing either excessive or insufficient fluid delivery, visible as white smoke or NOx spike.
Probable Causes
- Wiring harness damage: Chafed or corroded wires in the dosing unit harness, especially near the DEF tank or exhaust manifold, cause intermittent shorts.
- Faulty dosing unit: Internal electronic failure of the DEF dosing unit, such as a cracked pressure sensor diaphragm or stuck solenoid valve.
- ECM software glitch: Outdated or corrupted ECM calibration that misinterprets the dosing unit feedback signal, common after firmware updates.
- Poor ground connection: High-resistance ground at the dosing unit or ECM chassis ground point, introducing noise into the signal line.
Advanced Technical Analysis
The ECM monitors the dosing unit’s internal pressure sensor via a 0-5V analog input sampled at 100 Hz. When the signal varies by more than ±0.3V within 100 ms, the ECM flags FMI 2. This erratic behavior often stems from electromagnetic interference from nearby high-current cables or a failing sensor reference voltage regulator. In Deutz TCD 2.9 engines, this code appears after a hard reset of the ECM without a subsequent dosing unit relearn procedure.
Electrical analysis reveals that a 5V reference short to ground or battery voltage causes the signal to jump erratically. The ECM debouncing timer requires the fault to be present for 2.5 seconds before logging it. Real-world cases show that a loose pin in the 12-pin connector at the dosing unit—common on MAN D26 engines—produces intermittent contact that matches this timer threshold exactly, leading to intermittent fault logging.
When FMI 2 is active, the ECM enters a safety fallback: it disables DEF dosing entirely and sets a torque derate of 25% after 30 minutes of engine run time. The ECM also locks out active regeneration, forcing the DPF to passively regenerate only. This is a critical protection to prevent raw DEF from entering the exhaust, which could damage the SCR catalyst. Bosch ED17 ECUs on Mercedes-Benz OM471 engines use this strategy.
Long-term diagnosis should include a wiring integrity test using a 10A current loop to identify intermittent high-resistance joints. In workshops, this fault often reappears after a dosing unit replacement if the new unit is not calibrated via the OEM diagnostic tool. For example, on Volvo D13 engines, failing to perform the ‘Dosing Unit Adaptation’ procedure after replacement causes FMI 2 to return within 20 engine hours.
Step-by-Step Troubleshooting Guide
- Visual harness check: Inspect the dosing unit harness for chafing, corrosion, or loose connectors, especially near the DEF pump and exhaust manifold.
- Voltage reference test: Measure the 5V reference at the dosing unit connector; if unstable, backprobe the ECM pin to isolate the fault.
- Signal waveform capture: Use an oscilloscope to capture the dosing unit feedback signal during key-on; look for spikes above 5.5V or below 0V.
- ECM recalibration: Perform a dosing unit relearn procedure using the OEM diagnostic tool to reset the ECM’s signal baseline and clear the fault.
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