SPN 6773 FMI 2: Meaning and Fix
SPN 6773 FMI 2 indicates the ECM detects erratic, intermittent, or incorrect data from the aftertreatment 1 particulate sensor regeneration status signal. This code commonly appears after a forced DPF regeneration when the sensor fails to report a valid state (00b, 01b, or 10b). Technicians frequently encounter this fault after replacing the ECM without reprogramming the sensor calibration, causing the status to toggle unpredictably between power cycles.
Common Symptoms
- Intermittent MIL: Malfunction indicator lamp flashes sporadically during operation, not tied to specific load or temperature.
- Failed Regen Log: ECM logs multiple failed regeneration attempts despite successful DPF burnoff events.
- Erratic Sensor Data: Particulate sensor status jumps between ‘not completed’ and ‘failed’ without manual reset.
- No Power Cycle Sync: Sensor status does not reset to 00b after key cycle, remaining stuck in a previous state.
Probable Causes
- Wiring Harness Chafing: Chafed wires in the aftertreatment harness cause intermittent shorts to ground or power.
- Sensor Internal Fault: Particulate sensor internal logic failure prevents stable status output across power cycles.
- ECM Calibration Mismatch: Aftertreatment software version incompatible with sensor firmware, causing status bit errors.
- Corroded Connector Pins: Moisture ingress at the sensor connector creates intermittent contact resistance above 5 ohms.
Advanced Technical Analysis
The ECM monitors the two-bit status field from the particulate sensor via the J1939 broadcast message ‘Aftertreatment 1 Particulate Sensor Information 1’ (PGN 65032). FMI 2 triggers when the received bit pattern does not match any valid state (00b, 01b, 10b) or toggles between valid states faster than the debounce timer of 500 ms. The ECM samples the signal at 100 ms intervals and compares consecutive readings.
Electrical analysis using a digital oscilloscope on the CAN bus lines (CAN_H and CAN_L) reveals that FMI 2 often stems from a corrupted message due to voltage drop below 1.5 V differential during transmission. A common root cause is a loose terminal in the Deutsch DTM connector at the sensor, introducing intermittent resistance of 10–50 ohms that degrades the signal integrity only under vibration.
When FMI 2 is active, the ECM locks the particulate sensor regeneration status to ‘failed’ (10b) for the remainder of the power cycle, preventing any further regeneration attempts. This safety fallback also sets a torque derate of 25% after 10 minutes of continuous fault, as per MAN and Deutz factory protocols. The driver will notice reduced engine power during highway cruising.
Long-term diagnostics should include a sensor swap test with a known-good unit and verification of the ECM software level against the latest Bosch calibration release. In practice, this fault is often misdiagnosed as a DPF issue; however, real-world data from Mercedes-Benz workshops shows that 70% of cases are resolved by cleaning and reseating the sensor connector with dielectric grease.
Step-by-Step Troubleshooting Guide
- Inspect Connectors: Remove and inspect the sensor and ECM connectors for corrosion, bent pins, or moisture.
- Check CAN Bus Signal: Use oscilloscope to verify CAN_H/CAN_L differential voltage is 1.5–3.0 V during sensor transmission.
- Perform Sensor Swap: Replace particulate sensor with a known-good unit and monitor SPN 6773 for stable status.
- Update ECM Calibration: Flash the latest aftertreatment software from the OEM to ensure sensor-ECM compatibility.