What This FMI Means
FMI 21 indicates the data value is low in a way the module does not classify as a simple electrical voltage fault. This is distinct from FMI 4 (voltage low): FMI 21 is used when the parameter value is out of range in a direction that does not map to a straightforward circuit voltage condition.
FMI 21 applies to calculated values, network-received data, and parameters derived from multiple inputs — situations where the module cannot apply a simple "is this wire too low or too low?" test. The data is low-out-of-range, but the fault pathway is different from a sensor circuit voltage issue.
How It Appears With SPN Codes
FMI 21 appears on SPNs for parameters that are computed or network-received rather than directly measured by a simple analog voltage circuit. Calculated fuel consumption values, network-transmitted torque or speed values, and OEM-defined status parameters can all produce FMI 21 when the computed or received value is low out of range.
When FMI 21 appears on a network-received SPN, the transmitting module's fault code log is the starting point — the source of the low-range data value is usually apparent from the transmitting module's own active codes.
How to Approach Diagnosis
Identify whether the SPN is directly measured or network-received. For directly-measured SPNs, check whether the physical parameter is genuinely low or whether the sensor has drifted. For network-received SPNs, identify and check the transmitting module.
OEM service information is the authoritative source for what condition triggers FMI 21 on a specific SPN. The OEM documentation describes the specific threshold or logical condition that causes the module to log FMI 21 for that parameter — generic J1939 references describe the FMI's general meaning but not the OEM-specific trigger.
What Drivers Should Record
Record the full SPN/FMI, source address, all related codes, and operating conditions at the time the fault appeared. The source address is important because FMI 21 behavior depends on which module and which manufacturer's logic is involved.
Note any recent changes — software updates, module replacements, or component swaps — that preceded the appearance of FMI 21. Post-programming FMI 21 codes can indicate a calibration or configuration mismatch between modules sharing J1939 data.
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
How is FMI 21 (data low, non-electrical) different from FMI 4 (voltage low)?
FMI 4 specifically indicates the electrical signal voltage is low. FMI 21 indicates the reported data value is low, but the module does not classify it as a simple voltage-level fault. FMI 21 is used when the parameter value is out of range in a way that doesn't map to a straightforward high-voltage or low-voltage circuit condition — for example, a calculated or received data value that is low without the circuit itself being electrically faulty.
FMI 21 appears on a communication-received parameter — what does that mean?
When FMI 21 appears on an SPN whose value is received over the J1939 network (rather than measured directly by the receiving module), it means the data value in the received message is low of the expected range. The module receiving the data cannot distinguish whether the transmitting module's sensor is bad or the physical parameter is genuinely low. The source module's own fault codes provide better insight into the root cause.
Can FMI 21 be temporary, or does it indicate a persistent condition?
Like most FMIs, FMI 21 can be intermittent or persistent depending on the underlying cause. A transient operating condition — a momentary spike in a calculated value — may produce a brief FMI 21 that goes inactive when conditions normalize. A degraded sensor or module that consistently reports low values will produce a persistent active FMI 21. The active/inactive status and recurrence pattern help distinguish temporary from developing problems.