John Deere Engines SPN105FMI3 Fault Code: Manifold Air Temperature Signal Out of Range High
Also called Intake Manifold Air Temperature Signal Out of Range High, Manifold Air Temperature Input Voltage High
Manifold Air Temperature Signal Out of Range High · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 105 FMI 3 means the Manifold Air Temperature (MAT) sensor input voltage to the ECU is too high, above 4.9 volts (some applications list 4.8 volts), a level that corresponds to an impossibly low intake air temperature. The ECU falls back to a limp-home MAT value and disables the high manifold air temperature engine protection feature until the fault clears.
Medium severity. The engine will keep running using a substituted MAT value, but the high manifold air temperature protection feature is disabled while this code is active, so the ECU can no longer catch a real overheated intake air condition. Diagnose it within the shift rather than ignoring it long term.
What does John Deere Engines error code SPN105FMI3 mean?
SPN 105 FMI 3 is a John Deere engine fault code for the Manifold Air Temperature sensor circuit. The MAT sensor is a thermistor mounted in the intake manifold that measures intake air temperature. As a thermistor, its resistance changes with temperature, and that resistance change causes the input voltage seen by the ECU to vary: higher intake air temperatures produce lower MAT voltages, and lower temperatures produce higher voltages.
FMI 3 specifically flags a voltage signal that is out of range on the high side. The ECU sees a MAT input voltage above the sensor's high voltage specification, commonly 4.9 volts (one listing gives 4.8 volts for OEM engine applications). That voltage corresponds to a temperature colder than is physically possible inside a running intake manifold, so the ECU knows the signal itself is bad rather than trusting it as a real reading.
The ECU uses MAT input together with the manifold air pressure (MAP) sensor input to work out engine air flow, so a bad MAT signal affects more than just a temperature readout. Once this code sets, the ECU substitutes a fixed limp-home MAT value, either 60°C (140°F) or 50°C (122°F) depending on the control software version, and disables the high manifold air temperature engine protection feature until the fault is resolved.
What triggers a John Deere Engines SPN105FMI3 code?
The code sets when the manifold air temperature sensor circuit voltage rises above the high voltage specification, listed as 4.9 volts in most references and 4.8 volts for OEM engine applications in one listing. This voltage level corresponds to a temperature lower than what is physically possible for intake manifold air, which is how the ECU recognizes the signal as invalid rather than a genuine cold reading.
Common causes of SPN105FMI3
- Dirty, damaged, or poorly positioned terminals at the ECU connector (described variously as the 60-way or 30-way ECU connector depending on application)
- Dirty, damaged, or poorly positioned terminals at the MAT sensor connector
- An open or short in the wiring harness between the ECU and the MAT sensor, including a possible open in the MAT sensor ground circuit
- A faulty or failed MAT sensor itself
- A faulty ECU (least common, checked only after wiring and sensor are cleared)
How to troubleshoot John Deere Engines SPN105FMI3: first checks
- With the key off and connectors left connected, visually inspect the ECU connector and the MAT sensor connector for dirt, contamination, corrosion, damage, or terminals that are pushed back or poorly seated
- Inspect the wiring harness between the ECU and the MAT sensor for chafing, pinches, or damage that could cause an open circuit or a short to a higher voltage source
- Check for an open in the MAT sensor ground circuit, since an open input or open ground is the most probable cause of a high-voltage-out-of-range reading on this type of circuit
- If probing connectors is necessary, use the proper connector adapter test kit rather than forcing probes directly into the terminals, since forcing probes can damage the terminals and create a new fault
- After wiring and connectors check out, test the MAT sensor itself, and consider ECU integrity only after the sensor and harness are confirmed good
How the code clears
No separate clearing step is listed beyond correcting the underlying wiring, connector, or sensor problem. Once the MAT input voltage returns to a normal operating range, the ECU should stop substituting the limp-home value and re-enable the high manifold air temperature engine protection feature, and the blinking warning light tied to this code should go out on its own.
Frequently asked questions
What does SPN 105 FMI 3 mean on a John Deere engine?
It means the Manifold Air Temperature sensor's input voltage to the ECU is too high, above the sensor's high voltage specification (4.9 volts or 4.8 volts depending on application). That voltage corresponds to an intake air temperature colder than physically possible, so the ECU flags the signal as invalid.
Can I keep running the machine with this code active?
The engine can keep running because the ECU substitutes a fixed limp-home MAT value, either 60°C (140°F) or 50°C (122°F). However, the ECU's high manifold air temperature engine protection feature is disabled while the code is active, so it's best to diagnose and fix it rather than run indefinitely.
What usually causes SPN 105 FMI 3?
The most common causes are dirty, damaged, or poorly positioned terminals at the ECU or MAT sensor connectors, an open or short in the wiring harness between them, or an open in the MAT sensor ground circuit. A failed MAT sensor or, less commonly, a faulty ECU can also cause it.
Is this an electrical problem or a sensor problem?
It can be either. Because the circuit is designed to see rising voltage as ground resistance increases, an open input wire or open ground circuit is the more probable cause. A short from the sensor's 5-volt input wire to a higher voltage source is possible but less likely.
What should I check first if I get this code?
Start with a visual inspection of the ECU connector and MAT sensor connector for corrosion, contamination, or damaged terminals, without disconnecting them. Then check the wiring harness between the ECU and sensor for damage, and check the MAT sensor ground circuit for an open.
Will using a multimeter directly on the connector terminals cause damage?
Forcing test probes directly into connector terminals can damage them. The proper approach is to use a connector adapter test kit designed for this purpose so you don't create a new fault while diagnosing the original one.
Does this code affect other engine protection features?
Yes. While SPN 105 FMI 3 is active, the ECU's high manifold air temperature engine protection feature is disabled, since the ECU is relying on a fixed substitute MAT value instead of the real sensor reading.