FMI 4 Meaning in Truck Fault Codes

What This FMI Means

FMI 4 indicates the sensor circuit voltage is low of the module's expected range. This is a circuit-level fault — the measured voltage itself is wrong, not merely the parameter value. For most sensors, the module supplies a 5V reference to the sensor signal circuit and expects to read back a voltage between approximately 0.5V and 4.5V. FMI 4 means the voltage is below this window, often near 0V.

FMI 4 distinguishes from FMI 0 and FMI 1 (valid-data threshold faults) because FMI 4 means the circuit voltage itself is outside the electrical range — not just the parameter value. A sensor reading above 4.75V or below 0.25V on a 5V reference circuit is electrically invalid, regardless of what the actual physical parameter is doing.

How It Appears With SPN Codes

FMI 4 appears on most sensor-type SPNs — boost pressure (SPN 102), oil pressure (SPN 100), coolant temperature (SPN 110), exhaust gas temperature (SPN 2791), DEF quality (SPN 3031), and many others. It is one of the most frequently encountered FMI values in truck diagnostics.

Multiple simultaneous FMI 4 codes on unrelated sensors suggest a shared circuit issue rather than independent sensor failures. A common 5V reference supply fault, a shared signal ground fault, or a module output problem can produce FMI 4 across several SPNs at once.

The Sensor Disconnect Test

Disconnecting the sensor and measuring the circuit voltage at the harness connector identifies whether the fault is in the sensor or the wiring: if the voltage rises to the reference level with the sensor disconnected, the sensor itself has an internal short to ground. If the voltage stays low with the sensor disconnected, the fault is in the wiring or module. This test is fast, non-destructive, and avoids replacing a sensor only to find the harness is the problem.

Always use a high-impedance digital voltmeter and avoid forcing probes or jumpers into sealed weather-pack connectors. Measuring at the back of the connector or through a break-out box prevents pin damage. Compare the measured voltage against the OEM specification for the reference supply and expected return range.

What Drivers Should Record

Record the full SPN/FMI and whether the code is active or inactive. Note any related warning lamps and whether a derate or engine protection event occurred alongside it.

FMI 4 codes that go active during specific conditions — engine fully warm, immediately after startup, after hard braking, or in cold weather — help the technician narrow down whether the fault is temperature-related, vibration-related, or associated with a specific system state that changes load on the sensor circuit.

Related Fault Code 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 type of circuit fault most commonly produces FMI 4 (voltage low)?

FMI 4 typically results from a short to ground, an open in the supply wire, or a missing pull-up on the sensor circuit. This is a circuit-level fault — the signal is at the wrong voltage level, not just an out-of-range reading. The first diagnostic step is usually to check the wiring continuity and connector condition at the sensor and at the module input.

Can FMI 4 appear on a sensor that is physically damaged, or only from wiring problems?

Both. A failed sensor with an internally shorted or open element can produce the same voltage low reading as a wiring fault. Disconnecting the sensor and measuring the circuit voltage at the harness connector (with the sensor unplugged) tells you whether the fault is in the wiring or in the sensor itself — if the voltage is still abnormal with the sensor disconnected, the wiring or module is the source.

If FMI 4 appears on multiple sensors at the same time, what does that suggest?

Multiple simultaneous FMI 4 codes on unrelated sensors strongly suggest a shared circuit fault — a reference voltage supply line that is missing or shorted to ground, a common power or ground bus issue, or an ECM output that is not behaving correctly. It is statistically unlikely for several independent sensors to fail in the same way simultaneously.

FMI 4 Explained