SPN 2567 FMI 4: Meaning and Fix
SPN 2567 FMI 4 indicates the J1939 transport protocol connection management (TP.CM_BAM) signal voltage is below normal or shorted low. This fault typically appears after an ECM replacement or wiring repair if the CAN bus termination or shield is improperly grounded. Technicians often encounter this when a new ECU is installed without verifying the correct 120-ohm termination resistors, causing message assembly failures.
Common Symptoms
- ECM No Communication: ECM fails to broadcast BAM messages; other controllers report loss of multipacket data transfer capability.
- Intermittent Data Loss: Sporadic loss of J1939 messages during high bus load, especially during DPF regeneration or transmission shifting.
- Diagnostic Link Timeout: Diagnostic tool cannot complete a full scan; TP.CM_BAM packets are missing or corrupted.
- Engine Derate Active: ECM triggers a torque reduction to protect aftertreatment systems due to missing broadcast data.
Probable Causes
- Shorted CAN_H Wire: CAN_H line shorted to ground or battery negative, pulling the TP.CM_BAM signal below the recessive threshold.
- Faulty Termination Resistor: Missing or incorrect 120-ohm resistor at the end of the backbone causes signal reflections and low voltage.
- ECM Internal Failure: Transceiver inside the ECM has failed, clamping the CAN bus voltage to near zero volts.
- Corroded Connector Pin: Pin 17 (CAN_H) on the Deutsch connector corroded, increasing resistance and sinking the voltage low.
Advanced Technical Analysis
The ECM continuously monitors the CAN_H line voltage relative to CAN_L during recessive states. A recessive bit should measure 2.5V ±0.5V on CAN_H. If the ECM detects CAN_H below 1.5V for more than 200 ms, it sets SPN 2567 FMI 4. This debouncing timer prevents false triggers from electrical noise during engine start.
A short to ground on the CAN_H line creates a voltage divider with the termination resistors. With a 120-ohm resistor at each end, a hard short to ground pulls CAN_H to 0V, while CAN_L remains at 2.5V. This violates the 1.5V recessive threshold and locks the bus into dominant state, blocking all BAM messages.
When this fault is active, the ECM’s J1939 transport layer enters a fail-safe mode. It stops broadcasting multipacket messages such as aftertreatment status and engine configuration. The ECM then sets a torque derate of 25% to prevent uncontrolled operation without critical data, as specified in SAE J1939-21.
Long-term prevention requires verifying CAN bus termination with an ohmmeter across CAN_H and CAN_L at the diagnostic port (60 ohms with ignition off). In workshops, this fault frequently follows ECM replacement when the technician forgets to install the termination resistor in the harness. Always measure resistance before powering the new ECM.
Step-by-Step Troubleshooting Guide
- Measure CAN Resistance: With ignition off, measure resistance between CAN_H and CAN_L at the diagnostic connector; expect 60 ohms.
- Check CAN_H Voltage: Ignition on, measure CAN_H to ground; should be 2.5V ±0.5V. Below 1.5V indicates a short.
- Inspect Connector Pins: Remove ECM connector and visually check pin 17 (CAN_H) for corrosion, bent pins, or moisture.
- Isolate Bus Segments: Disconnect one ECU at a time while monitoring CAN_H voltage to locate the shorted node.