SPN 625 FMI 8: Meaning and Fix
This fault code indicates that the Engine Control Module (ECM) has detected an abnormal frequency, pulse width, or period on the proprietary network #1 broadcast, specifically the Stop-Start Broadcast message. In practice, this code commonly appears after a forced DPF regeneration or after replacing the ECM without proper configuration, causing the stop-start system to malfunction or fail to engage.
Common Symptoms
- Start-Stop Failure: Engine fails to auto-stop at idle; system remains permanently active or inactive.
- Erratic Idle: Engine speed fluctuates or hunts during idle phases due to corrupted broadcast timing.
- Warning Lamp: Amber or red stop-start warning lamp illuminated on the dashboard continuously.
- Battery Drain: Premature battery discharge observed as the system cannot manage alternator load correctly.
Probable Causes
- Wiring Fault: Shorted, open, or high-resistance wiring on the proprietary network #1 bus line.
- ECM Mismatch: Replacement ECM not flashed with correct stop-start broadcast parameters per OEM spec.
- Sensor Drift: Crankshaft or camshaft position sensor providing erratic timing signals to the ECM.
- Power Supply Noise: Unstable 5V or 12V supply to the ECM causing clock jitter in broadcast generation.
Advanced Technical Analysis
The ECM continuously monitors the proprietary network #1 for the stop-start broadcast message. When the received pulse width deviates by more than ±5% from the calibrated 100 ms period, FMI 8 sets. This is a signal integrity check, not a data content check, meaning the ECM sees a valid message but with incorrect timing.
Electrical breakdown analysis reveals that the ECM uses a hardware debouncing timer of 50 ms to validate each pulse. If consecutive pulses exhibit a period outside 95-105 ms for three cycles, the fault latches. Common root causes include a weak pull-up resistor on the bus or intermittent ground offset exceeding 200 mV.
Upon activation, the ECM initiates a safety fallback: it disables the stop-start function and sets a torque derate of 25% to protect the starter and battery. The broadcast message is still sent but flagged as invalid, preventing the transmission controller from initiating an auto-stop. This derate remains active until the fault is cleared.
Long-term prevention involves verifying bus termination resistors (120 ohms at each end) and ensuring the ECM firmware matches the vehicle’s build date. Technicians frequently encounter this fault after replacing the ECM without transferring the original calibration. A scope capture of the broadcast line should show clean square waves at 10 Hz with less than 2% jitter.
Step-by-Step Troubleshooting Guide
- Bus Inspection: Check proprietary network #1 wiring for shorts, opens, or corrosion using a multimeter.
- ECM Calibration: Verify ECM firmware version matches OEM stop-start broadcast parameters; reflash if needed.
- Sensor Check: Test crankshaft position sensor output signal for correct amplitude and frequency with an oscilloscope.
- Power Supply: Measure ECM supply voltage under load; ensure it remains between 11.5V and 14.5V without ripple.