SPN 906 FMI 31: Meaning and Fix
SPN 906 FMI 31 indicates the ECM detects a condition exists on the front axle right wheel relative speed signal, meaning the reported speed deviates abnormally from the vehicle reference speed. This code commonly appears after a forced DPF regeneration when the vehicle is stationary but the wheel speed sensor still outputs a false pulse, or after replacing the ECM without recalibrating the wheel speed parameters. The ECM continuously compares SPN 906 against SPN 904 (front axle speed) and triggers the fault if the difference exceeds a calibrated threshold for a defined debounce period.
Common Symptoms
- ABS Warning Lamp: Instrument cluster illuminates the ABS warning lamp immediately when the fault becomes active.
- Erratic Speedometer: Speedometer needle fluctuates or drops to zero intermittently during straight-line driving.
- Traction Control Reduced: Traction control system disables or intervenes incorrectly, especially on slippery surfaces.
- Cruise Control Disabled: Cruise control and adaptive cruise functions are inhibited by the ECM for safety.
Probable Causes
- Sensor Air Gap: Excessive air gap between the wheel speed sensor and tone ring due to bent spindle or worn bearing.
- Tone Ring Damage: Missing, chipped, or magnetized tone ring teeth causing erratic signal patterns.
- Wiring Harness Fault: Chafed, pinched, or corroded wiring between sensor and ECM, especially near the wheel well.
- ECM Calibration Error: Incorrect wheel circumference or axle ratio parameter programmed after ECM replacement or flash.
Advanced Technical Analysis
The ECM microcontroller monitors the right front wheel speed sensor (SPN 906) using a dedicated digital input capture unit. The signal is a variable reluctance or Hall-effect square wave. The ECM calculates the period between rising edges and compares it to the CAN bus reference speed from the driveline. When the difference exceeds 15% for more than 500 ms, the FMI 31 condition is set. This logic prevents false triggers from short transient disturbances like gravel roads.
Electrically, the sensor circuit includes a pull-up resistor to 5V and a series current-limiting resistor. The ECM monitors both the signal voltage and the DC offset. An open circuit causes the voltage to stay at 5V, while a short to ground pulls it to 0V. The FMI 31 condition is only set when the signal amplitude is within normal range but the frequency content is implausible, such as a 50% duty cycle indicating a stuck sensor.
Upon detecting this fault, the ECM activates a safety fallback: it disables the traction control system and sets a torque derate of 25% to prevent unintended wheel slip compensation. The ABS module also switches to a degraded mode using only diagonal wheel pairs. This is based on SAE J1939-71 and manufacturer-specific power train protection strategies documented in Bosch and MAN factory manuals.
Long-term prevention requires verifying tone ring runout with a dial indicator (max 0.2 mm) and checking sensor resistance (typically 1.2–2.0 kΩ). Technicians frequently encounter this fault after installing aftermarket wheel bearings that alter the air gap. A real-world workshop example: a 2020 Mercedes-Benz Actros repeatedly set this code after a tire change; the cause was a magnetized tone ring from a magnetic drill stand used during repair.
Step-by-Step Troubleshooting Guide
- Visual Inspection: Inspect sensor wiring and connector for corrosion, chafing, or loose pins at the ECM and sensor.
- Air Gap Check: Measure sensor-to-tone-ring gap with a feeler gauge; specification is 0.3–1.5 mm.
- Tone Ring Test: Rotate wheel slowly and use an oscilloscope to verify clean, uniform square wave pulses.
- Calibration Verify: Use diagnostic tool to confirm wheel circumference parameter matches tire size stamped on sidewall.