What causes a SPN 3921 code?

SPN 3921 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 3921 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 3921 active.

How do I fix SPN 3921?

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 3921

J1939 SPN 3921 — Body/Chassis parameter

What it means

J1939 Aftertreatment 1 Intake O2 Sensor - Circuit Fault / Body Controller Signal Error. Note: In some OEM contexts, this specifically refers to an interface error between the chassis and body controller regarding auxiliary equipment.