What causes a SPN 4991 code?

SPN 4991 can be triggered by several issues — see the verified probable causes and step-by-step diagnostics shared by mechanics and owners on OBDIY.

Can I drive with a SPN 4991 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4991 active.

How do I fix SPN 4991?

Follow the step-by-step diagnostic on this page, then read the verified fixes contributed by other Heavy Truck owners — the answer marked "Verified Cause" worked for the original poster.

OBDIY

J1939 SPN

SPN 4991

Cummins fault code 4991

What it means

Aftertreatment SCR Catalyst Conversion Efficiency - Data Valid But Below Normal Operating Range - Least Severe Level. This indicates the SCR system is not effectively reducing NOx emissions to the required levels.