Detailed guide for SPN 4794 FMI 31
Back to top ↑What this fault means
SPN 4794 FMI 31 indicates that the ECM or aftertreatment controller has detected a condition involving the SCR catalyst or associated system. The exact description and diagnostic strategy vary by OEM, so the code should be verified for the specific engine before parts are replaced.
A condition-exists FMI often describes the state the controller has detected rather than identifying one failed component.
What you may notice
Possible symptoms include aftertreatment warnings, emissions-system messages, inducement or derate, and related SCR communication or efficiency faults.
What to check first
Record all active and inactive aftertreatment codes. Inspect SCR-related wiring and connectors, especially anything recently disconnected during exhaust, transmission or chassis work.
Confirm that required aftertreatment components are physically present and correctly connected. Check network or module communication faults if the system is not reporting expected information.
Avoid assumptions
Do not assume the SCR catalyst itself has failed simply because the description mentions the catalyst system. A wiring problem, disconnected component, controller issue or another aftertreatment fault can make the system appear unavailable.
If emissions hardware has been removed, altered or bypassed, restore the vehicle to the applicable legal and OEM configuration.
Follow the manufacturer's diagnostic tree and verify communication, wiring and component presence before replacing expensive aftertreatment assemblies.
Mechanic community notes for SPN 4794 FMI 31
Back to top ↑Real-world tips from technicians. Submissions are moderated to keep spam and “my cousin fixed it with duct tape” content out.
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