SAE J1939 diagnostic codes for drivetrain components. Includes SPN and FMI fault codes for retarders, power take-off (PTO), transfer cases, axles, fan drives, cruise control, and driveline management systems on trucks and off-highway equipment.
Diagnostic Code SPN 84 FMI 8: Meaning and Fix SPN 84 FMI 8 indicates the ECM detects an abnormal frequency, pulse width, or period from the wheel-based vehicle speed sensor circuit. This fault often appears after a transmission overhaul or when a non-OEM speed sensor is installed, causing signal distortion. Technicians frequently encounter this after … Read more
Diagnostic Code SPN 84 FMI 21: Meaning and Fix SPN 84 FMI 21 indicates the wheel-based vehicle speed signal has drifted below the expected valid range for a sustained period. This fault commonly appears after a wheel speed sensor is replaced without proper air gap adjustment or when corrosion on the sensor connector increases resistance. … Read more
Diagnostic Code SPN 84 FMI 20: Meaning and Fix SPN 84 FMI 20 indicates the wheel-based vehicle speed signal has drifted abnormally high beyond calibrated plausibility thresholds defined in SAE J1939-71. The ECM receives a speed value exceeding the physically achievable range without corresponding powertrain confirmation. This fault commonly appears after a vehicle speed sensor … Read more
Diagnostic Code SPN 84 FMI 17: Meaning and Fix SPN 84 FMI 17 indicates that the wheel-based vehicle speed is valid but falls below the expected operational range. This issue often arises after tire replacements or when incorrect wheel speed sensor inputs are detected. In practice, technicians frequently encounter this fault after routine maintenance or … Read more
Diagnostic Code SPN 84 FMI 16: Meaning and Fix This fault indicates the wheel-based vehicle speed signal from the tailshaft or wheel speed sensor is above the normal operating range, typically exceeding the calibrated maximum. Technicians often see this after a forced DPF regeneration where the vehicle is driven at high speed on a dyno, … Read more
Diagnostic Code SPN 84 FMI 15: Meaning and Fix SPN 84 FMI 15 indicates the wheel-based vehicle speed signal is valid but exceeds the upper threshold of normal operating range, classified as least-severe overspeed condition per SAE J1939-71. The ECM receives speed data from the tailshaft or ABS wheel speed sensors and flags this fault … Read more
Diagnostic Code SPN 84 FMI 10: Meaning and Fix SPN 84 FMI 10 flags an abnormal rate of change in wheel-based vehicle speed calculations, often following extensive ECM updates or sensor replacement. Common in heavy-duty vehicles after sensor recalibration, this fault disrupts cruise control functionality. Technicians frequently encounter this fault code when there’s a discrepancy … Read more
Diagnostic Code SPN 597 FMI 13: Meaning and Fix The ECM detects that the brake switch signal voltage or timing does not match the expected calibrated range for pedal released or depressed states. This code commonly appears after a forced DPF regeneration when the brake pedal is held down for an extended period, causing the … Read more
Diagnostic Code SPN 1590 FMI 18: Meaning and Fix SPN 1590 FMI 18 indicates the Adaptive Cruise Control system is transmitting valid data but operating below normal parameters. This commonly occurs after ECM replacements when ACC calibration values haven’t been properly transferred, causing the system to report abnormally low operational states. Technicians frequently encounter this … Read more
Diagnostic Code SPN 598 FMI 31: Meaning and Fix SPN 598 FMI 31 signals that the clutch switch input to the ECM has entered a persistent error condition (binary 10b). This fault commonly appears after a forced DPF regeneration when the clutch pedal is not cycled correctly, or following ECM replacement without proper calibration. The … Read more