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 598 FMI 18: Meaning and Fix SPN 598 FMI 18 indicates the clutch switch signal is valid but below normal operating voltage thresholds, typically manifesting as persistent 00b state readings. This commonly occurs in fleet vehicles after aggressive clutch usage or moisture intrusion into switch assemblies. The ECM interprets continuous low voltage … Read more
Diagnostic Code SPN 598 FMI 14: Meaning and Fix SPN 598 FMI 14 indicates special instructions are needed for the clutch switch, a critical component of the cruise control and drivetrain systems. This fault often appears in vehicles where clutch pedal signals are inconsistent, such as after extensive clutch or transmission work. Ensuring the clutch … Read more
Diagnostic Code SPN 596 FMI 9: Meaning and Fix SPN 596 FMI 9 indicates the Engine Control Module (ECM) has detected an abnormal update rate from the Cruise Control Enable Switch signal on the J1939 bus. This means the switch state (enabled/disabled) is not being transmitted at the expected periodic interval. Technicians frequently encounter this … Read more
Diagnostic Code SPN 596 FMI 7: Meaning and Fix SPN 596 FMI 7 indicates the cruise control enable switch mechanism is not responding properly to driver input commands. This fault commonly appears after steering column repairs or when drivers report intermittent cruise control activation despite proper switch operation. The ECM detects inconsistent mechanical switch behavior … Read more
Diagnostic Code SPN 596 FMI 6: Meaning and Fix SPN 596 FMI 6 refers to a fault in the cruise control enable switch circuit, detected when the current is above normal or grounded. This issue can affect the ability to engage or maintain cruise control functionality. Technicians often encounter this fault code following ECM replacements … Read more
Diagnostic Code SPN 596 FMI 5: Meaning and Fix This fault indicates the ECM detects an open circuit or current below normal on the cruise control enable switch circuit (SPN 596 FMI 5). In practice, this often appears after a wiring harness repair or when the switch is unplugged during cab maintenance. The ECM expects … Read more
Diagnostic Code SPN 596 FMI 4: Meaning and Fix SPN 596 FMI 4 indicates the cruise control enable switch signal voltage has dropped below the ECM’s expected threshold, typically below 1.5V on a 5V reference circuit. This fault commonly appears after dashboard replacement or during wet weather conditions when moisture infiltrates switch connectors. The ECM … Read more
Diagnostic Code SPN 596 FMI 3: Meaning and Fix SPN 596 FMI 3 indicates a voltage above normal or a shorted high condition in the Cruise Control Enable Switch. This fault typically appears after wiring modifications or ECM replacements, leading to erratic cruise control behavior. Technicians often encounter this issue when performing maintenance or upgrades … Read more
Diagnostic Code SPN 596 FMI 2: Meaning and Fix SPN 596 FMI 2 indicates the cruise control enable switch signal is erratic, intermittent, or incorrect per SAE J1939. The ECM expects a steady 00b (disabled) or 01b (enabled) state. A floating voltage or rapid toggling triggers this fault. Technicians often see this after a forced … Read more
Diagnostic Code SPN 596 FMI 18: Meaning and Fix SPN 596 FMI 18 indicates the cruise control enable switch signal is below normal operating range, typically below 0.5V threshold. This fault commonly appears during highway operations when drivers attempt to activate cruise control but receive no system response. The ECM detects insufficient voltage from the … Read more