John Deere Engines SPN971FMI31 Fault Code: External Derate Commanded
Also called External Derate Switch Activated, External Engine Derate Switch Active, External Fuel Derate Switch Active, External Fuse Derate Switch Active
External Derate Commanded · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 971 FMI 31 sets when the ECU sees no input voltage from an OEM-installed external derate switch, meaning the switch has activated (or the circuit reading it has failed). The ECU responds by derating engine power, and on applications with the shutdown feature enabled, the engine will shut down 30 seconds after the code sets. This is a warning-level code, but the power loss and possible shutdown make it operationally serious.
High severity. The alarm level is listed as Warning, but the control response is not minor: the ECU derates power immediately, and on shutdown-equipped applications the engine stops running after 30 seconds. Treat this as a stop-and-diagnose issue rather than something to run through a shift.
What does John Deere Engines error code SPN971FMI31 mean?
SPN 971 FMI 31 refers to an external derate switch, an accessory that OEMs can wire into the ECU using programmable trim options. This is not a factory-standard John Deere sensor; it is a customer or OEM-installed switch tied to some external device, mechanical linkage, or environmental condition on the machine.
The switch is normally open. When whatever it monitors exceeds a set condition, the switch closes, grounding the voltage signal the ECU expects to see. When the ECU reads no input voltage on that circuit, it interprets this as the derate switch being activated and responds by cutting engine power.
Depending on how the OEM programmed the trim option, the response is either a straight percentage-based power derate, or a derate that ramps down over time. On applications with the shutdown feature enabled, losing input voltage on this circuit also starts a shutdown countdown. This code exists specifically to protect the machine or an external system the OEM decided was important enough to tie into engine power.
What triggers a John Deere Engines SPN971FMI31 code?
DTC 000971.31 sets when the ECU does not read an input voltage on the external derate switch circuit. Because the switch is normally open and closes to ground the signal when activated, a lost voltage reading looks the same to the ECU whether the switch legitimately closed or the circuit failed open or shorted.
Common causes of SPN971FMI31
- The external device, mechanical linkage, or environmental condition tied to the switch has genuinely activated it (this is a real trip, not necessarily a fault)
- Bad terminals or a bad connector at the external derate switch itself
- Bad terminals or connector at the ECU or at the C08 connector
- Open or shorted wiring in the harness or auxiliary harness, including pinched or melted wires
- Bad external fuel derate switch (failed internally)
- Bad connector connection at the ECU harness
- Outdated or bad ECU software
- Bad ECU
How to troubleshoot John Deere Engines SPN971FMI31: first checks
- Confirm whether the machine actually has an external derate switch installed and programmed through an OEM trim option; not all applications use this feature
- Check what the switch monitors (device, linkage, or environmental condition) and verify whether that condition is genuinely present before assuming an electrical fault
- Inspect the switch's terminals and connector for corrosion, looseness, or damage
- Trace the harness and auxiliary harness for open circuits, shorts, pinched wiring, or melted insulation between the switch and the ECU
- Check the ECU connector and the C08 connector for bad terminals or poor connections
- Verify ECU software is current, since outdated software is listed as a possible cause
How the code clears
No separate clearing procedure is listed for this code. Since the ECU sets it in real time whenever the ignition is on and the fault condition is active, correcting the wiring, connector, or switch issue (or resolving the actual condition that activated the switch) should allow the code to clear on its own. If it does not clear after repairs, recheck ECU software and connections at the ECU and C08 connector before suspecting the ECU itself.
Frequently asked questions
What does SPN 971 FMI 31 mean on a John Deere engine?
It means the ECU is not reading an input voltage from an OEM-installed external derate switch. This switch is normally open and closes to ground the signal when whatever it monitors exceeds a set condition, so the ECU treats a lost voltage reading as the switch being activated and derates engine power in response.
Will my engine shut down if this code sets?
On applications programmed with the shutdown feature, yes: the engine will shut down 30 seconds after the code sets. On applications with the derate feature instead, the engine will derate by 2% per minute until it reaches 20% of full power, without shutting down.
Is this a factory John Deere sensor or something added by the equipment maker?
It is an OEM-installed feature. The external derate switch is optional and only active if the equipment manufacturer programmed the trim option into the ECU, tying engine power to some external device, mechanical linkage, or environmental condition specific to that machine.
Why would the derate switch activate for no obvious reason?
The switch closes when a monitored condition is exceeded, so an activation can be a real, correct response to that condition. It can also be caused by a bad connector, bad switch, or an open or shorted harness wire that mimics the switch closing. Both possibilities need to be checked.
Can I keep operating the machine with this code active?
Not for long. The alarm level is Warning, but the ECU immediately begins derating power, and depending on programming the engine may shut down after 30 seconds or ramp down to 20% power over time. Diagnose it before continuing normal work.
Where should I start looking for the fault?
Start at the external derate switch itself and its connector, since bad terminals or a failed switch are common listed causes. Then check the harness and auxiliary harness for opens or shorts, and finally check the ECU connector, the C08 connector, and ECU software.
Does this code always mean there's an electrical fault?
No. It can also mean the switch is doing exactly what it was installed to do because the condition it monitors, whether mechanical, environmental, or another device, has genuinely been triggered. Confirming that condition is part of the diagnosis, not just chasing wiring.