Komatsu Engines 3561 Fault Code: Aftertreatment Diesel Exhaust Fluid Dosing Unit - Current Below Normal or Open Circuit
Aftertreatment Diesel Exhaust Fluid Dosing Unit - Current Below Normal or Open Circuit · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
Fault code 3561 (SPN 3361, FMI 5) means the ECM has detected an open circuit or below-normal current at the aftertreatment diesel exhaust fluid (DEF) dosing unit. When active, DEF injection into the aftertreatment system is disabled and, if left unaddressed for an extended period, engine torque will be reduced.
High severity. DEF dosing is disabled immediately, and continued operation with this fault active will trigger a torque reduction. It won't strand the machine instantly, but it should be diagnosed before the derate kicks in or before it triggers deeper aftertreatment issues.
What does Komatsu Engines error code 3561 mean?
Fault code 3561 (SPN 3361, FMI 5) points to the DEF dosing unit circuit, the pump and injector assembly that meters diesel exhaust fluid into the exhaust stream so the aftertreatment system can convert NOx into harmless byproducts. The controller sends the dosing unit a battery voltage supply, a ground, and a control signal that varies depending on the DEF pressure being commanded at the time.
FMI 5 specifically means the ECM is seeing current below normal or an open circuit on that dosing unit circuit, essentially the controller is calling for dosing action and not seeing the electrical response it expects.
When this fault is active, the ECM disables DEF injection entirely. It will also light the amber CHECK ENGINE lamp and/or malfunction indicator lamp (MIL) right away. If the engine keeps running with the fault active for an extended period, expect a torque reduction as the system protects itself from running an incomplete emissions cycle.
Common causes of 3561
- Open circuit in the aftertreatment diesel exhaust fluid dosing unit itself
- Open circuit in the aftertreatment diesel exhaust fluid controller harness (wiring between the controller and the dosing unit)
- Malfunctioning aftertreatment diesel exhaust fluid controller
How to troubleshoot Komatsu Engines 3561: first checks
- Pull up the fault code on the electronic service tool and confirm it's active or inactive, note whether the CHECK ENGINE lamp or MIL is illuminated
- Visually inspect the wiring harness between the DEF controller and the dosing unit for chafing, corrosion, pinched sections, or a disconnected connector
- Check the OEM Power Distribution Center fuses and relays for loose, missing, or intermittent connections, this is specifically flagged as a common cause of intermittent power and datalink issues with aftertreatment components
- Reference the OEM wiring diagram for your specific machine before probing any OEM-supplied wiring, since dosing unit location and harness routing are OEM dependent
- With the harness confirmed intact, test for continuity and proper voltage/ground at the dosing unit connector, and if wiring checks out, suspect the dosing unit or the controller itself
How the code clears
To validate a repair, perform a key cycle, then start the engine and let it idle for 1 minute. The fault code status on the electronic service tool will change to INACTIVE immediately once the diagnostic runs and passes, and the ECM will turn off the amber CHECK ENGINE lamp at that same time. On OBD-equipped engines, the MIL will only turn off after 3 consecutive trips where the diagnostic runs and passes. The service tool's 'Reset All Faults' command can be used to clear active and inactive faults and to extinguish the MIL on OBD applications.
Frequently asked questions
What does Komatsu fault code 3561 mean?
It means the ECM has detected an open circuit or below-normal current at the DEF (diesel exhaust fluid) dosing unit. This is SPN 3361 with FMI 5. DEF injection is disabled while the fault is active.
Can I keep driving or working with fault code 3561 active?
You can operate in the short term, but DEF dosing is disabled the whole time, and if you run the engine for an extended period with the fault active, the ECM will reduce engine torque. It's best treated as a diagnose-soon issue, not one to ignore.
Where is the DEF dosing unit located on my machine?
The dosing unit's location is OEM dependent, it varies by machine and application. Check your OEM service manual for the exact location and wiring diagram for your equipment.
What usually causes SPN 3361 FMI 5?
The three documented causes are an open circuit in the dosing unit itself, an open circuit in the controller-to-dosing-unit harness, or a malfunctioning DEF controller. Loose or corroded Power Distribution Center fuses and relays are also called out as a common source of intermittent aftertreatment faults.
Why won't the check engine light turn off after I fixed the wiring?
The lamp status changes to INACTIVE and the CHECK ENGINE lamp turns off as soon as the diagnostic runs and passes after a key cycle and about a minute of idle. On OBD-equipped engines, though, the MIL specifically needs three consecutive trips where the diagnostic passes before it goes out.
How do I manually clear fault code 3561 after repair?
Use the 'Reset All Faults' command on the electronic service tool. It clears both active and inactive faults and will extinguish the MIL on OBD applications once the underlying issue is actually fixed.
Is fault code 3561 an electrical problem or a DEF quality problem?
This specific code is electrical, an open circuit or low current condition in the dosing unit's control circuit. It's not about DEF fluid quality or contamination, it's about the wiring, connector, dosing unit, or controller not completing the circuit as expected.