John Deere Engines SPN31 Fault Code: Cylinder 1 EUI Fault
Cylinder 1 EUI Fault · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 31 sets when the ECU detects an open or a short in the cylinder number 1 electronic unit injector (EUI) circuitry on John Deere 10.5 L and 12.5 L engines. Depending on the cause, cylinder 1 may misfire, not fire at all, or the engine may run rough. If codes 032 and 033 are also active, the engine will run on cylinders 4, 5, and 6 only.
High severity. The engine can still run with this code active, but cylinder 1 may not be firing correctly, which causes a miss and lost power. If it combines with codes for cylinders 2 and 3, half the engine shuts down. This should be diagnosed before continued heavy-load operation.
What does John Deere Engines error code SPN31 mean?
SPN 31 refers to the electronic unit injector for cylinder number 1. On these 10.5 L and 12.5 L engines, each cylinder has its own EUI, which combines the injection pump and injector in one unit mounted in the cylinder head under the valve cover, spraying directly into the center of the cylinder bore.
The ECU controls injection timing and fuel quantity by energizing and de-energizing a solenoid in each EUI's valve housing, which opens and closes the EUI's spill valve. Power to the EUIs for cylinders 1, 2, and 3 comes from one common wire, while cylinders 4, 5, and 6 share a separate common wire. The ECU switches each individual injector by opening and closing its own ground circuit.
When SPN 31 is active, the ECU does not change how it controls the rest of the system. But depending on what caused the fault, cylinder 1 may not fire at all, the engine may develop a noticeable miss, or there may be a dead short in that injector's circuit. If codes for cylinders 2 and 3 (Engine 032 and 033) are also set alongside 031, that points to a wiring problem on the shared power feed to cylinders 1, 2, and 3, and the ECU will not turn any of those three injectors on, running the engine only on cylinders 4, 5, and 6.
Common causes of SPN31
- An open circuit in the cylinder number 1 EUI wiring or connector.
- A short circuit in the cylinder number 1 EUI wiring or connector.
- A shared wiring problem on the common power feed to cylinders 1, 2, and 3 if codes 032 and 033 are also set along with 031, since these three injectors share one power wire.
How to troubleshoot John Deere Engines SPN31: first checks
- Check the engine for a miss, rough running, or reduced power that would point to cylinder 1 not firing.
- Check whether Engine codes 032 and 033 are also active. If they are, focus on the common power wiring feeding cylinders 1, 2, and 3 EUIs rather than treating this as an isolated cylinder 1 problem.
- Inspect the cylinder 1 EUI wiring harness and connector terminals for corrosion, looseness, chafing, or damage. Use a proper connector adapter test kit to probe terminals; never force test probes directly into connector terminals, since this damages them.
- Check the individual ground circuit for the cylinder 1 EUI solenoid, since the ECU controls each injector by switching this ground circuit.
- With the engine off, check EUI circuit continuity and resistance to help isolate an open versus a short, and compare cylinder 1 readings against the other cylinders if possible.
How the code clears
No separate clearing or reset procedure is listed for this code. Once the open or short in the cylinder number 1 EUI circuitry is found and repaired, the code should stop being active on the next key cycle or drive cycle where the fault condition is no longer detected.
Frequently asked questions
What does John Deere SPN 31 mean?
It means the ECU has detected an open or a short in the wiring or circuitry for the cylinder number 1 electronic unit injector (EUI) on a 10.5 L or 12.5 L engine.
Can I keep driving or working with SPN 31 active?
The engine can keep running, but cylinder 1 may not be firing properly, causing a miss or power loss. If codes 032 and 033 are also active, the engine will only be running on cylinders 4, 5, and 6, which is a significant loss of power and should be addressed before continuing heavy work.
Why would cylinder 1 stop firing but the engine still runs?
Each cylinder on these engines has its own electronic unit injector, and the ECU controls each one independently by switching its ground circuit. A fault limited to cylinder 1 does not automatically shut down the other cylinders unless the shared power wiring for cylinders 1, 2, and 3 is affected.
Why do codes 031, 032, and 033 sometimes appear together?
Cylinders 1, 2, and 3 share a common power wire feeding their EUIs. If all three codes set together, it points to a problem in that shared wiring rather than three separate injector failures, and the ECU will disable all three injectors, running the engine on cylinders 4, 5, and 6 only.
What tools should I use to check the EUI connector?
Use a proper connector adapter test kit designed for these connectors. Forcing standard test probes directly into the connector terminals can damage them and create a new problem.
Is there a specific reset procedure for SPN 31?
No reset procedure is listed for this code. Repairing the open or short in the cylinder 1 EUI circuit should allow the fault to clear on its own once the condition is no longer detected.
How do I know if it's an open circuit or a short circuit?
This requires checking the wiring and connector for cylinder 1's EUI circuit with the engine off, looking at continuity and resistance, and comparing findings to a known-good cylinder circuit where possible.