What causes a SPN 4123 code?

SPN 4123 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 4123 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4123 active.

How do I fix SPN 4123?

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 4123

Cummins fault code 4123

What it means

Aftertreatment 1 SCR Catalyst System Missing Tank Level Signal or SCR System Efficiency - Specifically relating to NOx conversion efficiency being below a required threshold.