SPN 2567 FMI 7: Meaning and Fix
SPN 2567 FMI 7 indicates that the Transport Protocol Connection Management (TP.CM_BAM) message sequence failed because the addressed ECU did not respond within the expected time window. This code commonly appears after a controller replacement or when a gateway module is offline, preventing multi-packet J1939 messages from being assembled correctly.
Common Symptoms
- Missing diagnostic data: Scan tool cannot retrieve complete DM1 or DM2 messages from certain ECUs on the bus.
- Intermittent communication loss: Engine or transmission data appears and disappears randomly during operation.
- Delayed parameter updates: Dashboard gauges freeze or update slowly when multiple ECUs transmit large data sets.
- Failed component calibration: Injector or aftertreatment system calibration fails due to incomplete data transfer.
Probable Causes
- ECU power supply fault: Intermittent voltage drop on the target ECU prevents it from sending TP.CM_CTS frames.
- CAN bus termination error: Missing or incorrect termination resistor causes signal reflections that corrupt BAM handshake.
- Corrupted transport layer: Internal TP state machine in the source or destination ECU hangs due to software glitch.
- Physical wiring damage: Chafed or corroded CAN_H or CAN_L wires cause intermittent connection breaks during multi-packet transmission.
Advanced Technical Analysis
The ECM’s transport layer monitors the time between the BAM (Broadcast Announce Message) and the first CTS (Clear To Send) response. If the CTS is not received within 200 ms (per SAE J1939-21), the ECM sets SPN 2567 FMI 7 and aborts the session. This timeout is critical for multi-packet messages like ECU identification or calibration data.
Electrically, FMI 7 points to a mechanical or timing failure rather than a signal short. The debouncing timer inside the CAN controller resets each time a valid frame is detected; however, if the target ECU’s transceiver enters bus-off state due to excessive errors, the CTS never arrives. Technicians often see this after a 12V-to-24V system conversion without proper level shifting.
When this fault activates, the ECM usually does not enter a torque derate but may disable certain diagnostic services. For example, a forced DPF regeneration may be blocked because the aftertreatment controller cannot receive the full 8-packet command. The ECM logs the failed session and attempts a retry up to three times before locking the fault.
Long-term prevention requires verifying that all ECUs share a common ground reference and that the CAN bus is terminated with 120 ohms at both ends. In workshops, this fault is frequently resolved by updating the gateway controller firmware to handle extended IDs correctly, or by replacing a failing battery that causes intermittent ECU resets during high-current loads like starter engagement.
Step-by-Step Troubleshooting Guide
- Verify bus termination: Measure resistance between CAN_H and CAN_L at the diagnostic port; must be 60 ohms.
- Check target ECU power: Monitor voltage at the non-responding ECU connector during key-on and engine crank.
- Inspect wiring integrity: Perform a continuity test on CAN_H, CAN_L, and shield wires for breaks or shorts.
- Cycle ECU power: Disconnect battery for 30 seconds to reset all transport layer state machines and retry.