SPN 5746 FMI 9: Meaning and Fix
This fault indicates the ECM is not receiving the expected periodic status message from the Aftertreatment 1 DEF Dosing Unit 1 Heater Relay. The expected update rate has been violated, often due to intermittent wiring faults or a failing relay driver. Technicians frequently encounter this code after a forced DPF regeneration, when thermal stress on connectors temporarily disrupts the signal.
Common Symptoms
- MIL Illumination: Red or amber malfunction indicator lamp activates within one drive cycle after fault detection.
- No DEF Dosing: Dosing unit heater remains off, preventing DEF thawing in cold ambient conditions below -7°C.
- Engine Derate: ECM initiates a progressive torque reduction after 30 minutes of continuous fault presence.
- Frozen DEF Line: Operator may report DEF line blockages or pump cavitation during winter operation.
Probable Causes
- Open Heater Relay: Internal relay coil open circuit prevents ECM from detecting relay state changes.
- Corroded Connector: Pin oxidation at the relay or ECM connector introduces intermittent contact resistance.
- ECM Driver Failure: Internal ECM output driver damaged due to short circuit or overcurrent event.
- Wiring Harness Chafing: Insulation wear on the relay control wire causes intermittent ground or open circuits.
Advanced Technical Analysis
The ECM monitors the relay status message on the J1939 bus at a fixed interval, typically 100 ms. An abnormal update rate occurs when the message is missing for more than 1.5 seconds. The microcontroller sets FMI 9 and logs a snapshot of the bus load and relay command state for post-event analysis.
Electrical analysis reveals that the relay coil typically draws 0.5–1.5 A. A 50% duty cycle PWM control signal from the ECM is used for soft-start. If the ECM detects a missing feedback pulse due to a stuck relay armature or open flyback diode, the debouncing timer expires and the fault is stored.
Upon activation of FMI 9, the ECM enters a safety fallback where the heater relay is permanently de-energized. The aftertreatment controller then commands a 25% torque derate to prevent DEF crystallization in the dosing unit. The derate escalates to 50% if the fault persists beyond two hours of engine runtime.
Long-term prevention requires inspecting the relay socket for thermal damage, especially after DPF regeneration events. In MAN and Mercedes-Benz applications, replacing the relay with a sealed automotive-grade component (e.g., TE Connectivity V23074) and applying dielectric grease to the connector has proven to eliminate recurrence in over 90% of field cases.
Step-by-Step Troubleshooting Guide
- Verify Message Rate: Use J1939 scan tool to monitor relay status PGN 64950; check for missing or delayed frames.
- Measure Relay Coil: With ignition off, measure resistance across relay coil pins; spec is 20–80 ohms.
- Inspect Connector: Remove and visually inspect relay and ECM connectors for corrosion, bent pins, or fretting.
- Check ECM Output: Probe ECM control pin with oscilloscope; expect a 12V PWM signal during heater request.