The Failure Mode Identifier (FMI) is one of the four components of a complete SAE J1939 Diagnostic Trouble Code (DTC). While the SPN (Suspect Parameter Number) identifies which component has failed, the FMI defines exactly how it failed — whether the signal is too high, too low, erratic, or the circuit has an open or short condition.
The J1939 standard defines 32 FMI codes (0–31). Of these, 22 have active technical definitions and 10 (FMI 22–30, excluding 31) are reserved for future SAE assignment. Use the decoder below to look up any FMI instantly.
FMI must be a number between 0 and 31.
All 32 FMI definitions — click any row to decode
| FMI | Name | Category |
|---|
Understanding FMI codes is essential for accurate J1939 diagnostics. A technician who misreads the FMI may replace a perfectly functional sensor when the actual fault is a wiring short, or vice versa. The FMI narrows the diagnostic path before a single measurement is taken.
FMI 3 and FMI 4 always point to electrical circuit faults — voltage high or voltage low — and require a multimeter and wiring diagram before any component is replaced. FMI 7 indicates a mechanical non-response, meaning the actuator received a valid command but failed to execute it — pointing to a stuck valve, worn actuator, or mechanical obstruction. FMI 9 and FMI 19 are network-specific codes that indicate CAN bus communication problems, requiring a J1939 protocol analyzer rather than standard electrical tests.
FMI 0, 1, 15, 16, 17, and 18 form a graduated severity scale for out-of-range data. FMI 0 and 1 represent the most severe thresholds and typically trigger immediate torque derate. FMI 15, 16, 17, and 18 represent progressively less severe conditions that allow the ECM to issue staged warnings before escalating to full protection mode.
FMI 20 and 21 — data drifted high and data drifted low — are often overlooked but highly valuable. Unlike the threshold-based FMI 0/1, these codes indicate a gradual sensor drift over time, which almost always points to sensor aging, contamination, or a calibration offset rather than a wiring fault.
For a complete diagnostic guide on any specific SPN and FMI combination, use the SPN FMI Lookup tool or browse fault codes by system category.