John Deere Engines SPN84FMI31 Fault Code: Vehicle Speed Mismatch
Also called ECU Calculated Vehicle Speed and CAN Vehicle Speed Mismatch, ECU Calculated Vehicle Speed and CAN bus Vehicle Speed Mismatch
Vehicle Speed Mismatch · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 84 FMI 31 sets when the wheel speed reported to the ECU by the Basic Control Unit (BCU) via a Pulse Width Modulated signal does not match the wheel speed reported by the Basic Informator (BIF) over CAN. John Deere lists this as a Vehicle Speed Mismatch fault. Engine performance may be affected slightly, but the machine is generally driveable while you diagnose it.
Medium severity. John Deere states engine performance may be affected slightly when this code sets. It is not described as a shutdown or immediate safety event, but a speed sensing disagreement should be diagnosed promptly since vehicle speed data can affect other control functions.
What does John Deere Engines error code SPN84FMI31 mean?
SPN 84 FMI 31 means the engine control unit (ECU) is receiving two different vehicle speed values that should agree but don't. One value comes from the BCU as a Pulse Width Modulated (PWM) signal, generated from an inductive wheel speed sensor mounted on the rear axle. As axle teeth pass the sensor, they create AC signals whose frequency is proportional to wheel speed. The BCU converts this into a PWM signal sent to both the BIF and the ECU.
The second value reaches the ECU over the CAN bus, originating from the BIF. Both the PWM-based reading and the CAN-based reading are supposed to match. When they don't, the ECU flags DTC 84.31.
John Deere notes that if DTC 1069.31 is present at the same time as 84.31, you should diagnose 1069.31 first, since it may be the root cause of the mismatch.
What triggers a John Deere Engines SPN84FMI31 code?
The code sets when the wheel speed value delivered to the ECU from the BCU via the PWM signal does not match the wheel speed value delivered to the ECU from the BIF over the CAN bus. No specific voltage, time, or percentage threshold is given for how much mismatch is required to trigger the fault.
Common causes of SPN84FMI31
- Bad terminals or connectors at the ECU, BIF, or BCU connectors
- A CAN bus communication error affecting the speed signal reaching the ECU
- An open or short in the circuit between BCU terminal 64 and terminal 21L at the BIF connector
- Tire size incorrectly programmed in the BIF, which throws off the calculated wheel speed
- An open or short in the circuit between terminal D2 at the ECU and terminal 64 at the BCU
- A bad ECU
How to troubleshoot John Deere Engines SPN84FMI31: first checks
- Check for any co-occurring DTC 1069.31. If present, diagnose that code first, since John Deere specifically calls this out as the priority fault.
- Inspect all connectors at the ECU, BIF, and BCU for corrosion, loose pins, or damaged terminals. A bad connection at any of these three points can cause a mismatch.
- Verify the tire size programmed in the BIF matches the actual tires installed. An incorrect tire size setting will make the calculated wheel speed disagree with the CAN-reported value.
- Check the wiring between BCU terminal 64 and BIF terminal 21L for opens or shorts.
- Check the wiring between ECU terminal D2 and BCU terminal 64 for opens or shorts.
- Scan for other active CAN bus fault codes that might point to a broader communication problem before condemning the ECU.
How the code clears
No separate clearing step is listed. Since this is a comparison fault between two live sensor/network inputs, it should clear once the wiring, connector, tire size programming, or CAN bus issue causing the mismatch is corrected and the two speed readings agree again. Follow the application troubleshooting manual and technical manual referenced by John Deere for the full step-by-step diagnostic procedure, since detailed pinpoint tests are not included in this summary.
Frequently asked questions
What does SPN 84 FMI 31 mean on a John Deere engine?
It means the ECU is seeing two different vehicle speed values that should match but don't: one from the BCU sent as a Pulse Width Modulated signal, and one from the BIF sent over the CAN bus. John Deere calls this a Vehicle Speed Mismatch.
Is it safe to keep driving with SPN 84 FMI 31 active?
John Deere states that engine performance may be affected slightly when this code sets. It is not flagged as an immediate shutdown condition, but you should get it diagnosed soon since it points to a wiring, connector, or CAN bus issue that could worsen or affect other systems.
What is the first thing to check for this code?
Check whether DTC 1069.31 is also active. John Deere specifically instructs diagnosing 1069.31 first if it appears alongside 84.31. After that, inspect connectors at the ECU, BIF, and BCU, and verify the tire size programmed in the BIF is correct.
Can a wrong tire size setting cause this fault?
Yes. John Deere lists an incorrectly programmed tire size in the BIF as one of the possible causes, since it changes the calculated wheel speed value compared to what the wheel speed sensor and BCU are actually measuring.
Which wiring circuits are specifically called out for this code?
John Deere lists two circuits to check: between BCU terminal 64 and BIF terminal 21L, and between ECU terminal D2 and BCU terminal 64. An open or short in either circuit can cause the speed mismatch.
Does this code mean the ECU is bad?
A bad ECU is listed as a possible cause, but it's last on John Deere's list. Bad connectors, CAN bus errors, wiring opens/shorts, and incorrect tire size programming are all more common causes to rule out first.
Is there a specific reset procedure for SPN 84 FMI 31?
No separate clearing step is listed. The fault should clear on its own once the underlying mismatch between the BCU PWM signal and the BIF CAN signal is resolved.