SAE J1939 fault codes for vehicle electrical and power systems. Includes SPN and FMI diagnostic codes for alternators, batteries, vehicle electrical power modules, DC/DC converters, DC/AC inverters, and power distribution on commercial vehicles.
Diagnostic Code SPN 802 FMI 20: Meaning and Fix SPN 802 FMI 20 identifies a condition where the Relay Diagonal 1 feedback signal has drifted above its expected threshold, indicating an out-of-range voltage state within the H-bridge relay diagnostic circuit. The ECM continuously monitors relay diagonal feedback for plausibility against commanded states. This fault commonly … Read more
Diagnostic Code SPN 802 FMI 19: Meaning and Fix The SPN 802 FMI 19 code is triggered when the ECM detects erroneous data from the Relay Diagonal 1 over the network. This is common after electrical component replacements or harness repairs, leading to unexpected relay behavior. Technicians often encounter this code post-ECM replacement or wiring … Read more
Diagnostic Code SPN 802 FMI 17: Meaning and Fix SPN 802 FMI 17 indicates the Relay Diagonal 1 output voltage is below the normal operating range but data is still valid. This commonly appears after a battery disconnect or jump-start event where a voltage spike damages the relay driver. Technicians often see this after ECM … Read more
Diagnostic Code SPN 802 FMI 16: Meaning and Fix SPN 802 FMI 16 identifies a moderately severe above-normal condition on Relay Diagonal 1, a monitored output channel within the ECM’s relay bridge circuit. This diagonal path is part of a dual-diagonal relay topology commonly used in Bosch MSA/EDC control architectures for fail-safe switching. Technicians frequently … Read more
Diagnostic Code SPN 802 FMI 15: Meaning and Fix SPN 802 FMI 15 indicates that the Relay Diagonal 1 data is valid but exceeds its normal operating range, albeit with the least severity. This fault often surfaces following ECM replacements or updates, as the relay may temporarily exceed expected parameters during initialization. This situation may … Read more
Diagnostic Code SPN 802 FMI 10: Meaning and Fix SPN 802 FMI 10 (Relay Diagonal 1) indicates the ECM detected an abnormal rate of change in the relay output circuit, meaning the commanded state (ON/OFF) did not transition within the expected debounce window. This fault commonly appears after a forced DPF regeneration or when a … Read more
Diagnostic Code SPN 3597 FMI 8: Meaning and Fix SPN 3597 FMI 8 indicates the ECU Power Output Supply Voltage #1 has detected an abnormal frequency, pulse width, or period on its internal 5V or 12V switched output. This fault commonly appears after a forced DPF regeneration when the ECM’s internal power stage overheats, causing … Read more
Diagnostic Code SPN 3597 FMI 21: Meaning and Fix SPN 3597 with FMI 21 refers to a voltage drift issue in the ECU’s first power output. This fault often surfaces after extended periods of heavy electrical load, such as prolonged use of auxiliary systems. Technicians may encounter this problem following the installation of high-demand aftermarket … Read more
Diagnostic Code SPN 3597 FMI 20: Meaning and Fix This fault indicates the ECU’s internal power output supply #1 voltage has drifted above the calibrated high limit, often due to a failing voltage regulator or a shorted sensor supply. In practice, this code commonly appears after a forced DPF regeneration when thermal stress damages the … Read more
Diagnostic Code SPN 3597 FMI 19: Meaning and Fix SPN 3597 FMI 19 identifies a condition where the ECU Power Output Supply Voltage #1 message received over the J1939 datalink contains erroneous or unverifiable data. Per SAE J1939-71, FMI 19 specifically flags network-transmitted values that fail plausibility checks within the receiving control unit. This fault … Read more