What This FMI Means
FMI 24 indicates the data is within a technically valid electrical range but is not appropriate for the current operating context, or conflicts with other data the module has. The module is reporting a plausibility or state-consistency fault rather than a simple out-of-range or circuit fault — the data does not make sense given what the module knows about the current system state.
Unlike FMI 0 or FMI 1 (simple threshold exceedances), FMI 24 requires the module to evaluate multiple inputs simultaneously. The same sensor reading might be acceptable during one operating mode and flagged as FMI 24 during another, because the acceptable range for the parameter depends on the current operating state.
How It Appears With SPN Codes
FMI 24 appears on SPNs where the module performs cross-parameter plausibility checks. Examples include intake air temperature SPNs compared against coolant temperature and ambient temperature (to check plausibility at startup), exhaust temperature SPNs compared against engine load and speed, and transmission input speed SPNs compared against engine speed during engaged operation.
On diesel aftertreatment systems, FMI 24 on SCR inlet or outlet temperature SPNs can indicate the measured temperatures are inconsistent with the engine's operating state — suggesting either a sensor issue or an aftertreatment problem that is causing temperatures to be lower or higher than expected for the given engine load.
How to Approach Diagnosis
Identify what operating context triggered FMI 24. Because this fault is state-dependent, knowing the operating conditions at fault set time — engine speed, load, coolant temperature, ambient temperature, system mode — is essential for understanding which plausibility check failed.
Compare the suspect SPN's reading to related parameters using a live data display. A sensor that reads normally in isolation but conflicts with related parameters during specific operations may be physically correct but not sampling the right condition — a sensor installed in the wrong location, exposed to a different air pocket, or not properly thermally coupled to the measured medium.
What Drivers Should Record
Record the full SPN/FMI, all active and inactive codes present at the same time, and operating conditions — vehicle speed, load, terrain, temperature, and any operational events such as recent cold-weather starts, high-altitude operation, or system mode transitions.
Note whether FMI 24 appears consistently at a specific operating condition or at random. A code that consistently appears at a specific speed, load, or temperature narrows down which plausibility check is failing and gives the technician specific conditions to reproduce during diagnosis.
Related Pages
Sources
- SAE J1939 Standards Collection SAE International · official · accessed 2026-05-05 · confidence medium
Source: SAE International, SAE J1939 Standards Collection. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - NHTSA Manufacturer Communications Search National Highway Traffic Safety Administration · government · accessed 2026-05-05 · confidence high
Source: National Highway Traffic Safety Administration, NHTSA Manufacturer Communications Search. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
What does FMI 24 mean in plain terms?
FMI 24 indicates the data is technically within a valid range but is not appropriate for the current operating condition or conflicts with other data the module has. It is a plausibility or state-consistency fault rather than a simple out-of-range or circuit fault. The ECM is saying the data doesn't make sense given everything else it knows about the system's current state.
Can FMI 24 come from a sensor that is perfectly functional?
Yes. If a sensor reports a value that is electrically valid but physically implausible — for example, an intake air temperature sensor reading ambient temperature on a fully warmed engine that has been running at highway speed — the ECM may log FMI 24 as a plausibility fault. The sensor itself is measuring correctly, but the reading conflicts with related parameters. This often indicates a sensor that has drifted or a sensor that is not exposed to the correct physical input.
Is FMI 24 related to the conditions under which the fault occurred, or only to the signal itself?
FMI 24 is context-dependent — the module evaluates the data against the vehicle's operating state. The same sensor reading might be acceptable during cold startup but flagged as FMI 24 during a hot engine run. This is why recording operating conditions when the fault occurred helps diagnose FMI 24 more efficiently than a static code alone.