SPN 1067 FMI 31: Meaning and Fix
SPN 1067 FMI 31 indicates the Brake Signal Sensor 1 circuit has detected a continuous fault condition, often a stuck signal or internal sensor failure. This code commonly appears after an ECM software update or when the brake pedal is held partially depressed during key-on, confusing the sensor’s self-test routine. Technicians frequently encounter this after replacing the ECM without recalibrating the brake pedal position sensor.
Common Symptoms
- No Brake Light: Brake lights fail to illuminate when pedal is pressed, due to missing signal from sensor 1.
- Retarder Lockout: Engine retarder or exhaust brake is disabled by ECM for safety when brake signal is invalid.
- Cruise Disable: Cruise control automatically disengages and will not re-engage until fault is cleared.
- Dash Warning: Amber warning lamp illuminates with a ‘Brake Signal’ text message on the cluster display.
Probable Causes
- Sensor Stuck High: Internal Hall-effect sensor stuck in active state due to mechanical binding or debris inside pedal assembly.
- Open Supply Wire: 5V reference or ground circuit open at sensor connector, causing erratic or no output signal.
- ECM Calibration: Missing or incorrect brake pedal calibration after ECM replacement or software flash.
- Harness Chafing: Worn insulation on sensor harness near pedal bracket causes intermittent short to chassis ground.
Advanced Technical Analysis
The ECM continuously monitors the brake sensor 1 voltage on a dedicated analog input. Under normal conditions, the signal should transition from 0.5V (released) to 4.5V (fully pressed). FMI 31 is triggered when the ECM detects a constant voltage outside the expected range for more than 2 consecutive seconds, indicating a stuck or failed sensor. The diagnostic timer resets only after a valid transition is seen.
Electrically, the sensor is a 3-wire Hall-effect device powered by a regulated 5V supply from the ECM. FMI 31 often results from a loss of ground reference, causing the output to float near 5V. Using a digital multimeter, technicians should verify supply voltage at pin 1 (5V ±0.2V), ground continuity at pin 2 (<0.1Ω), and signal voltage at pin 3 while cycling the pedal.
When this fault is active, the ECM enters a safety fallback mode: it sets the brake signal to ‘released’ and disables all torque-based retarders and cruise control. Engine torque is derated by up to 40% to prevent unintended acceleration. The ECM will not allow a DPF regeneration to start while this fault is present, as brake switch input is required for stationary regen.
Long-term prevention requires proper pedal calibration after any ECM or sensor replacement. In MAN trucks, the calibration procedure involves a specific pedal sweep sequence stored in the ECU. A real-world example: a fleet of Deutz-powered excavators repeatedly logged SPN 1067 FMI 31 after pressure washing; water ingress into the pedal connector was the root cause. Sealing the connector with dielectric grease resolved the issue permanently.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Check pedal assembly for debris, broken return spring, or bent bracket that could hold sensor active.
- Supply Check: Measure 5V reference and ground at sensor connector with ignition on; repair open circuits if missing.
- Signal Sweep: Monitor signal voltage while slowly pressing and releasing pedal; look for smooth 0.5V to 4.5V transition.
- Calibration: Perform OEM-specific brake pedal calibration using diagnostic tool; clear fault and test drive.