What causes a SPN 2521 code?

SPN 2521 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 2521 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 2521 active.

How do I fix SPN 2521?

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 2521

J1939 SPN 2521 — Aftertreatment/Emissions parameter

What it means

SCR Outlet NOx sensor heater circuit failure. The NOx sensor cannot reach operating temperature to provide accurate data.