What an SPN/FMI pair means — and what it doesn't
On heavy-duty vehicles a fault code commonly pairs an SPN (which parameter or component) with an FMI (what kind of problem, such as out of range or open circuit). It tells you where the controller saw a problem, not which part has failed.
Reading the pair
The SPN identifies the signal or system; the FMI describes the failure type. The same SPN with different FMIs can point to very different faults — a wiring open versus a real out-of-range reading.
What it doesn't establish
A code doesn't prove a sensor is bad. Wiring, connectors, power or ground supply, or the condition being measured can all set the same code. Meaning can also vary by engine maker and software.
Next steps
Record the source controller, occurrence count and freeze-frame data, then follow the maker's troubleshooting tree.
Common questions
- Is SPN/FMI the same as an OBD-II P-code?
- No. They're different code systems. Light-duty vehicles typically use P, B, C and U codes.
Sources & review status
Written from general engineering principles. Modern source citations are pending editorial review.
Modern diagnostic terminology; not in the historic books. No SPN definitions are listed here because meanings depend on the maker.
Original Repairius explanation. Not yet reviewed by a qualified technician; not a substitute for the maker's service information.