What causes a SPN 6559 code?

SPN 6559 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 6559 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 6559 active.

How do I fix SPN 6559?

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 6559

Cummins fault code 6559

What it means

SCR Catalyst System - Out of Calibration. This code indicates that the NOx reduction efficiency has dropped below the required threshold, specifically pointing toward physical plugging or efficiency loss in the SCR.