What causes a SPN 921 code?

SPN 921 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 921 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 921 active.

How do I fix SPN 921?

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 921

J1939 SPN 921 — Engine Management parameter

What it means

SPN 921 typically refers to the Engine Cruise Control Resume Switch. It indicates a signal error or irrationality in the operator input circuit for the cruise control system.