John Deere Engines SPN655FMI6 Fault Code: Injector #5 Circuit Has Low Resistance
Also called Cylinder #5 EI Circuit Shorted, Cylinder #5 EUI Circuit Shorted, Electronic Unit Pump #5 Circuit Has Low Resistance, Injector #5 Spill Valve Circuit Has Low Resistance, The ECU detects an open in the Cylinder #5 EI circuit.
Injector #5 Circuit Has Low Resistance · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 655 FMI 6 means the ECU has detected low resistance or a short in the cylinder #5 fuel injection circuit, whether that's an electronic unit injector (EUI), electronic injector (EI), or electronic unit pump. Cylinder #5 will not fire when this code is active. The ECU will keep trying to control the engine normally, but you're running on fewer cylinders until it's fixed.
Medium severity. The engine keeps running and the ECU attempts normal control, but cylinder #5 is dead, meaning rough running, power loss, and possible long-term harm to injector or ECU components if a wire-to-wire short is left unaddressed. Diagnose within the shift rather than ignoring it.
What does John Deere Engines error code SPN655FMI6 mean?
SPN 655 FMI 6 covers several closely related fuel systems depending on engine family. On 10.5 L and 12.5 L engines it points to the cylinder #5 electronic unit injector (EUI) circuit, where the EUI is both a high-pressure injection pump and injector combined, mounted in the cylinder head under the valve cover. On 8.1 L engines it's the cylinder #5 electronic injector (EI) circuit fed from a high-pressure common rail. On other engines covered here, it's the cylinder #5 electronic unit pump circuit or the cylinder #5 electronic injector circuit on 6.8L/4.5L engines.
In every version, the ECU supplies a shared high-voltage feed to a group of injectors or pumps (cylinders 1-2-3 on one common wire, 4-5-6 on another) and controls each cylinder individually by switching its low side, or ground, on and off. FMI 6 means the ECU is seeing low resistance, essentially a short, on the cylinder #5 low-side circuit. This is distinct from a short-to-ground condition.
When the code sets, cylinder #5 will not fire. The ECU keeps trying to run the engine normally on the remaining cylinders, so the truck or machine won't necessarily shut down, but expect a noticeable loss of smoothness and power.
What triggers a John Deere Engines SPN655FMI6 code?
The ECU sets this code when it detects low resistance, or a short, specifically on the cylinder #5 low-side (ground) circuit for the injector, EUI, EI, or unit pump. On the electronic injector version, the fault can also be checked during a Harness Diagnostic Mode Test in Service ADVISOR, but only when fuel rail pressure is below 5 MPa (725 psi). The code is active while the engine is cranking or running under the fault condition.
Common causes of SPN655FMI6
- Bad terminals or connector at the ECU
- Bad terminals or connector at the EUI/EI/injector harness connector or at connector C01
- Bad #5 EUI, EI, injector, or unit pump itself (internal coil windings shorted together, causing low coil resistance)
- Bad EUI/EI/injector wiring harness, or bad unit pump harness
- Open or short in the harness between the ECU and the injector/EUI/unit pump connector, including the main harness or engine harness
- Open or short in the 90V circuit (EI version)
- Wire-to-wire short between the low-side wiring of two injectors or two unit pumps, which will show up as active low-resistance codes on both cylinders at once
- Bad ECU software
- Bad ECU
How to troubleshoot John Deere Engines SPN655FMI6: first checks
- Visually inspect the 48-way ECU connector, and on EUI systems the EUI harness connector at the back of the cylinder head, for dirty, damaged, or poorly seated terminals. On EI systems check the injector harness connector at the side of the cylinder head instead.
- On EI-based systems also inspect the 12-way (6.8L) or 6-way (4.5L) connector for corrosion, damage, or loose pins.
- Trace the wiring between the ECU and the cylinder #5 injector or unit pump connector for chafing, pinches, or exposed wire that could cause a short, especially anywhere that could bring the cylinder #5 low-side wire into contact with another cylinder's wire.
- Check for a second active low-resistance code on another cylinder in the same group (1-2-3 or 4-5-6). If two codes set together, suspect a wire-to-wire short between those two circuits rather than two failed injectors.
- If wiring and connectors check good, test the resistance of the #5 injector, EUI, or unit pump coil itself to look for internally shorted windings.
- If the component and harness both check out, suspect the ECU internals or, on some systems, outdated ECU software.
How the code clears
No separate clearing procedure is listed for this code. Once the underlying short or low-resistance condition is repaired, whether that's a connector, a harness short, or a failed injector or unit pump, the code should stop setting on its own during normal engine operation. If a scan tool is available, confirm the code is no longer active after the repair and a subsequent engine run.
Frequently asked questions
What does SPN 655 FMI 6 mean on a John Deere engine?
It means the ECU has detected low resistance, essentially a short, in the cylinder #5 fuel injection circuit. Depending on the specific engine, this could be the electronic unit injector (EUI), electronic injector (EI), or electronic unit pump circuit. Cylinder #5 will not fire while the code is active.
Will the engine still run with SPN 655 FMI 6 active?
Yes. The ECU will attempt to control the engine in a normal manner, but cylinder #5 will not fire, so expect rough running and reduced power rather than a shutdown.
Can a bad connector cause this code instead of a bad injector?
Yes. Bad terminals or connectors at the ECU, at the injector or EUI harness connector, or at connector C01 are all listed as possible causes, and should be checked before replacing the injector or unit pump.
Why would two cylinders show this code at the same time?
A wire-to-wire short between the low-side wiring of two injectors or two unit pumps will cause active low-resistance DTCs on both circuits simultaneously. If cylinder #5 and another cylinder in the same wiring group both show a similar code, check for chafed or shorted wiring between them before condemning either injector.
Is this the same as a short-to-ground fault?
No.
Does fuel rail pressure matter for diagnosing this code?
On electronic injector systems, this code can also be checked during a Harness Diagnostic Mode Test in Service ADVISOR, but only when fuel rail pressure is below 5 MPa (725 psi).
What should I inspect first before pulling the injector?
Start with a visual inspection of the ECU connector and the injector or EUI harness connector for dirt, damage, or poor terminal positioning, then trace the harness between the ECU and the cylinder #5 connector for wear or shorts before testing or replacing the injector or unit pump.