John Deere Engines SPN644FMI2 Fault Code: External Speed Command Input Erratic
Also called External Speed Command Input (OEM, Marine Only)
External Speed Command Input Erratic · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 644 FMI 2 means the ECU is seeing an erratic or intermittent tachometer sync signal (in Dual Engine Synchronization, marine applications) or erratic CAN data for an external engine speed or torque command. On marine dual-engine setups, synchronization drops out and the follower engine returns to normal throttle control. On CAN-based external speed command applications, the CAN white light turns on.
Medium severity. This is logged as a Warning-level condition. The ECU keeps trying to control the engine normally, but on marine dual-engine setups you lose synchronized throttle operation between the two engines, and on CAN-based speed command setups you lose reliable external speed control input. Neither is an immediate stop-the-engine emergency, but it should be diagnosed before extended operation, especially in marine service where engine sync affects vessel handling.
What does John Deere Engines error code SPN644FMI2 mean?
SPN 644 FMI 2 covers two related situations depending on the application. On marine engines using Dual Engine Synchronization, one engine's ECU is set up as Lead and the other as Follower. The Follower ECU listens to the Lead engine's tachometer output signal so both engines run in step. If that tachometer signal becomes erratic or intermittent, the code sets on the Follower ECU, which is the one receiving the signal.
On other applications, an external controller sends engine speed or torque requirements to the ECU over the CAN network. CAN is simply the communication bus that lets the various controllers on a machine or vessel talk to each other. If the ECU receives erratic or incorrect data through CAN for the speed or torque request, this code sets and the CAN white light turns on.
In both cases, the underlying issue is a signal integrity problem, not a request for a specific speed or torque value. The ECU is not able to trust what it's being told about desired engine speed.
Common causes of SPN644FMI2
- Radiated or conducted electrical noise from some part of the engine disrupting the tachometer sync signal between the Lead and Follower ECUs
- Loose electrical ground or power connections at any harness connector
- Loose electrical ground or power connections at alternator connections
- Loose electrical ground or power connections at frame and battery ground connections
- Electromagnetic interference (EMI) from an incorrectly installed 2-way radio
- Interference from a radar source
- Corrosion, dirt, or paint buildup causing intermittent or noisy electrical connections
- Erratic or incorrect CAN data being sent to the ECU from an external controller supplying engine speed or torque requirements
How to troubleshoot John Deere Engines SPN644FMI2: first checks
- Inspect all harness connectors for corrosion, dirt, or paint buildup that could cause a poor or noisy connection
- Check alternator connections for looseness or corrosion at both ground and power terminals
- Verify frame and battery ground connections are clean, tight, and free of corrosion
- Confirm any 2-way radio installed on the vessel or machine is mounted and grounded correctly, since improper installation is a known EMI source
- Consider whether a radar source nearby could be interfering with wiring runs
- On applications using CAN for external speed command, check CAN wiring and connectors, and verify the external controller sending speed or torque data is functioning correctly
How the code clears
No separate clearing step is listed. Once the erratic or intermittent tachometer signal (marine sync) or erratic CAN data is corrected at its source, the fault condition should stop being active. On marine dual-engine setups, expect synchronization to remain disabled and normal throttle control to stay in effect on the Follower engine until the issue is resolved and synchronization is re-engaged.
Frequently asked questions
What does SPN 644 FMI 2 mean on a John Deere marine engine?
It means the Follower ECU in a Dual Engine Synchronization setup is receiving an erratic or intermittent tachometer signal from the Lead engine's ECU. When this happens, synchronization is disabled and the Follower engine returns to normal, independent throttle control.
Why did my dual-engine sync stop working?
If SPN 644 FMI 2 has set, electrical noise, a loose ground or power connection, corrosion, or interference from something like an incorrectly installed 2-way radio or nearby radar is likely disrupting the tachometer signal between the two engine ECUs.
Is SPN 644 FMI 2 an emergency shutdown code?
No. It's logged as a Warning, and the ECU continues to control the engine normally. You lose the specific feature involved, either dual-engine synchronization or the external speed/torque command input, but the engine keeps running.
What is the CAN white light and why did it come on?
On applications where an external controller sends engine speed or torque requests over CAN, the CAN white light comes on when the ECU receives erratic or incorrect CAN data for that request. This is the CAN-based variant of SPN 644 FMI 2.
What should I check first for this code?
Start with the basics: all harness connectors, alternator connections, and frame and battery ground connections. Look for corrosion, dirt, or paint on terminals, and confirm any 2-way radio is installed and grounded correctly.
Can a 2-way radio really cause this fault?
Yes. An incorrectly installed 2-way radio is specifically listed as a possible source of electromagnetic interference that can make the tachometer sync signal or CAN data erratic enough to trigger this code.
Will fixing the wiring clear the code automatically?
No separate clearing procedure is listed for this code. Once the erratic signal or CAN data issue is corrected at its source, the condition should no longer be active, though you may need to re-engage synchronization on marine dual-engine setups.