SPN 155 FMI 9: Meaning and Fix
SPN 155 FMI 9 indicates the Engine Control Module (ECM) has detected an abnormal update rate from a connected controller or sensor, meaning the expected periodic message on the SAE J1939 bus is missing or irregular. This code commonly appears after a forced DPF regeneration when the ECM is temporarily overloaded, or after replacing the ECM without properly re-flashing the software. Technicians frequently encounter this fault on trucks with aftermarket electronic accessories that disrupt CAN bus timing, leading to intermittent power loss and warning lamps.
Common Symptoms
- Intermittent Power Loss: Engine may suddenly lose power or surge due to missing control signals from the affected node.
- Check Engine Lamp: Amber warning lamp illuminates on the dashboard, often accompanied by a stop engine lamp.
- Erratic Sensor Readings: Dash displays unstable values for engine speed, temperature, or pressure from the failing node.
- Failed OBD Readiness: Diagnostic tool may fail to communicate with certain ECUs or show incomplete monitor status.
Probable Causes
- CAN Bus Termination: Missing or incorrect 120-ohm termination resistors at bus endpoints cause signal reflection and timing errors.
- ECM Software Mismatch: Incompatible firmware version after ECM replacement or flash update disrupts message scheduling.
- Faulty Sensor Module: Internal failure of a J1939 node (e.g., engine speed sensor) stops sending periodic data frames.
- Corroded Connector: High resistance at the CAN connector due to moisture ingress or corrosion degrades signal integrity.
Advanced Technical Analysis
The ECM monitors the expected reception interval (typically 10–100 ms per J1939) for each message identifier. When a message fails to arrive within a defined debounce window (e.g., 1.5x the nominal period), the FMI 9 is set. This logic prevents false positives from transient glitches. In practice, a single missed frame can trigger the code if the ECM is configured for strict timing compliance, common in Deutz and MAN engines.
Electrical analysis reveals that abnormal update rates often stem from excessive bus capacitance or unterminated stubs. Using an oscilloscope, the differential voltage levels must be between 1.5 V and 3.5 V with a clean rectangular waveform. A slow rise time ( > 50 ns ) indicates a capacitance issue, typically from long unshielded wiring or multiple passive nodes. Bosch manuals recommend checking the CAN_H and CAN_L resistance at the diagnostic port (should be 60 ohms).
Upon detecting the abnormal update rate, the ECM enters a safety fallback mode. For engine-related nodes, the ECM may substitute a default value (e.g., fixed injection timing) and activate a torque derate of 25–50%. This protects the engine from operating with invalid data. Mercedes-Benz factory guidance states that the derate remains active until the missing message is restored and the fault is cleared via a diagnostic tool or 10 continuous valid cycles.
A long-term diagnostic strategy involves logging all CAN bus traffic with a J1939 analyzer during a road test. Real-world workshops have traced this fault to a failing aftertreatment control module (ACM) that intermittently dropped off the bus after thermal cycling. Prevention includes using dielectric grease on all CAN connectors and ensuring proper grounding of the shield wire only at one end. Reflashing the ECM with the latest OEM calibration often resolves software-related update rate issues.
Step-by-Step Troubleshooting Guide
- Verify CAN Termination: Measure resistance between CAN_H and CAN_L at the diagnostic port; must be 60 ohms.
- Inspect Connectors: Disconnect and inspect all CAN bus connectors for corrosion, bent pins, or moisture.
- Check ECM Software: Compare ECM part number and firmware version against OEM specifications; reflash if mismatched.
- Monitor Bus Traffic: Use a J1939 scan tool to identify which node is missing its periodic message.