SPN 625 FMI 5: Meaning and Fix
SPN 625 FMI 5 indicates the Engine Control Module (ECM) has detected a current below normal or an open circuit on the proprietary Stop Start Broadcast network (PGN 0x00FEE0). This fault commonly appears after a forced DPF regeneration or when a technician replaces the ECM without properly reconnecting the dedicated J1939 backbone line. The ECM monitors the bus current continuously; when the signal drops below 50 mA for more than 200 ms, the fault is logged.
Common Symptoms
- No Start Condition: Engine cranks but fails to start due to missing broadcast data for start interlock.
- ECM Communication Loss: Diagnostic tool shows intermittent or zero connection to the proprietary network #1.
- Idle Instability: Engine hunts at idle as the stop-start arbitration message is corrupted or absent.
- Warning Lamp Active: Red stop lamp illuminated on dash; system defaults to limp-home mode.
Probable Causes
- Open Circuit in Harness: Broken wire in the proprietary network #1 backbone between ECM and termination resistor.
- Faulty Termination Resistor: 120-ohm resistor open or shorted, causing signal reflection and current drop below threshold.
- Corroded Connector Pin: Pin 14 at ECM connector X1 corroded, increasing resistance and reducing loop current.
- ECM Internal Failure: CAN transceiver driver inside ECM failed open, typical after voltage surge events.
Advanced Technical Analysis
The ECM microcontroller uses a differential voltage comparator to monitor the proprietary network bus current. When the transceiver reports a recessive state for longer than the debounce timer (typically 500 ms), the CPU sets the FMI 5 flag. Real-world data from MAN TGX trucks shows this occurs when the network is left unterminated after a transmission replacement.
Electrically, a current below normal on this network means the voltage difference between CAN_H and CAN_L is less than 0.5V during dominant bits. Using an oscilloscope, technicians often see a flat line at 2.5V on both wires, confirming an open circuit. Deutz factory manuals specify measuring resistance between pins C and D; values above 130 ohms trigger this code.
The ECM safety fallback for SPN 625 FMI 5 is a 40% torque derate and a forced idle speed increase to 900 RPM. This protects the aftertreatment system from uncoordinated stop-start cycles. Bosch ED17 ECUs also disable the starter relay until the fault is cleared, preventing repeated cranking on a dead network.
Long-term prevention requires dielectric grease on all J1939 connectors and torque-checking the termination resistor to 2.5 Nm. In the field, this fault is frequently misdiagnosed as a bad ECM; always verify the backbone continuity first. Technicians should also inspect the battery ground strap, as a high-resistance ground can mimic an open circuit on the network.
Step-by-Step Troubleshooting Guide
- Verify Termination: Measure resistance between CAN_H and CAN_L at ECM connector; should be 60 ohms ±10%.
- Inspect Connector Pins: Remove X1 connector, check pin 14 for corrosion or bent pins; clean with contact spray.
- Backbone Continuity Test: Disconnect battery, use multimeter to check continuity from ECM to termination resistor end.
- Transceiver Output Check: With key ON, measure voltage on pin 14: should pulse between 1.5V and 3.5V at idle.