Cummins fault code 3679 is logged by the electronic control module (ECM) on Cummins ISX11.9 and ISX15 CM2250 engines when it detects a DEF dosing unit problem recorded as Aftertreatment Diesel Exhaust Fluid Dosing Unit – Root Cause Not Known. This page in the DiagTechPro manuals library explains the SPN, FMI and lamp behaviour so a technician can plan the repair before the truck reaches the bay. Every entry in this library follows the same layout, so a workshop can move quickly from the dashboard warning to the correct wiring or component test.
What Cummins fault code 3679 means
Cummins identifies this event as SPN 3361, FMI 11, with an amber warning lamp. The ECM turns on the amber CHECK ENGINE lamp when the diagnostic fails.
How the ECM sets this code
The ECM monitors the aftertreatment diesel exhaust fluid dosing unit and sets this code when a fault is detected but a more specific test has not isolated the root cause. Reading the exact SPN and FMI with a Cummins INSITE electronic service tool is the fastest way to confirm you are chasing the right circuit rather than a related aftertreatment code.
Possible causes and shop talk
The Cummins service information lists the following possible causes:
- Short or open circuit in the aftertreatment wiring harness
- Malfunctioning diesel exhaust fluid dosing unit
Inspect the wiring and connectors for the affected circuit first, then test the component itself, and only replace parts after INSITE confirms the failure. Check the calibration revision history on Cummins QuickServe Online in case a software update addresses the complaint.
Clearing Cummins fault code 3679
Because fault code 3679 can share root causes with other SPN 3361 aftertreatment and engine codes, repair the confirmed fault, then restore the monitored conditions and check that the status changes to inactive in INSITE. This document was last revised on 11-Jun-2010 by Cummins. Open the full step-by-step tree from the DiagTechPro manuals library, and confirm current bulletins through official Cummins engine support before finishing the job.
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