What causes a SPN 721 code?

SPN 721 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 721 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 721 active.

How do I fix SPN 721?

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 721

J1939 SPN 721 — Engine Management parameter

What it means

This code indicates a failure or irrational signal from the Secondary Engine Speed Sensor (typically the Camshaft or Transmission Output speed sensor used for back-up).