John Deere Engines SPN723FMI2 Fault Code: Engine Speed Input Noise
Engine Speed Input Noise · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 723 FMI 2 means the engine speed sensor signal has become erratic while the backup pump speed sensor signal is still valid. The ECU switches to the pump speed sensor to keep the engine running, but the switch can cause a momentary hesitation. If SPN 191 FMI 2 sets at the same time, the engine will die and not restart until one of the two codes is fixed.
High severity. The engine keeps running on the backup pump speed sensor and may only hesitate briefly, so this is not an immediate shutdown on its own. But if it combines with SPN 191 FMI 2, the engine stalls and will not restart, which makes this a stop-and-fix issue rather than something to ignore.
What does John Deere Engines error code SPN723FMI2 mean?
SPN 723 FMI 2 is set by the engine control unit when the primary engine speed sensor input turns erratic or noisy while the pump speed sensor input remains valid and usable. The engine speed sensor is an inductive pickup mounted in the timing gear cover that reads teeth on the camshaft drive gear, generating an AC signal whose frequency rises and falls with engine speed. The ECU normally relies on this signal as its main way of knowing how fast the engine is turning.
When the signal from the engine speed sensor becomes unreliable, the ECU falls back on the pump speed sensor to keep tracking engine speed so the engine can keep running. This handoff can cause the engine to hesitate momentarily while the ECU changes sources, but under normal circumstances the engine should continue running.
The bigger concern is if SPN 191 FMI 2, the pump speed sensor's own fault code, sets at the same time. With both speed inputs faulted, the ECU has no reliable speed signal left, and the engine will die and refuse to restart until at least one of the two sensor problems is repaired.
Common causes of SPN723FMI2
- Dirt, damage, or poorly seated terminals on the ECU 30-way connector or the engine speed sensor connector
- Bad harness connectors anywhere in the speed sensor circuit
- Bad alternator connections
- Bad chassis ground connections or a poor battery ground connection
- Corrosion, dirt, or paint causing intermittent, noisy electrical connections
- Wiring harness problems such as intermittent open or short circuits, particularly in the engine speed sensor wiring
- Electromagnetic interference (EMI) from an incorrectly installed 2-way radio
- Interference from a radar source
How to troubleshoot John Deere Engines SPN723FMI2: first checks
- Inspect the ECU 30-way connector and the engine speed sensor connector for dirt, damage, corrosion, or terminals that are pushed back or poorly seated
- Check harness connectors along the speed sensor circuit for corrosion, dirt, or paint intrusion that could cause noisy connections
- Inspect alternator connections and chassis/battery ground connections for looseness or corrosion, since poor grounds are a common source of electrical noise
- Wiggle-test and visually trace the engine speed sensor wiring harness for chafing, pinch points, or signs of intermittent open or short circuits
- If the machine has a 2-way radio installed, verify it was installed correctly and is not a source of electromagnetic interference
- Rule out nearby radar sources as a possible cause of signal interference
- Use the JT07328 Connector Adapter Test Kit to probe connectors rather than forcing probes directly into terminals, which can damage them
How the code clears
No separate clearing step is listed. Since the fault is tied to erratic sensor signal quality, correcting the wiring, connector, ground, or interference issue causing the noise should allow the code to clear on its own once the engine speed sensor signal is stable again. If SPN 191 FMI 2 is also active, that code must be repaired as well before the engine will restart.
Frequently asked questions
What does SPN 723 FMI 2 mean on a John Deere engine?
It means the ECU detected an erratic or noisy signal from the primary engine speed sensor while the pump speed sensor signal remained valid, so the ECU switched to the pump speed sensor to keep tracking engine speed.
Will the engine keep running with SPN 723 FMI 2 active?
Usually yes. The engine may hesitate momentarily during the switch to the pump speed sensor input but should continue running normally afterward.
Can this code cause the engine to stall and not restart?
Only if SPN 191 FMI 2, the pump speed sensor fault, sets at the same time. With both engine speed inputs faulted, the engine will die and won't restart until at least one of the two codes is repaired.
What usually causes SPN 723 FMI 2?
Common causes include dirty, damaged, or poorly positioned terminals on the ECU or engine speed sensor connectors, bad harness connectors, poor alternator or ground connections, corrosion or paint on connections, wiring shorts or opens in the speed sensor circuit, and electrical interference from a 2-way radio or radar source.
Is it safe to probe the connectors directly to diagnose this code?
No. Forcing probes into connector terminals can damage them. Use a proper connector adapter test kit, such as the JT07328, to make measurements without damaging terminals.
Does electromagnetic interference really trigger this code?
Yes, an incorrectly installed 2-way radio or interference from a radar source is listed as a possible cause of the erratic engine speed sensor signal.
How is SPN 723 FMI 2 cleared?
No separate clearing procedure is listed. Fixing the underlying wiring, connector, grounding, or interference issue that is causing the noisy signal should resolve the code.