What causes a SPN 3940 code?

SPN 3940 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 3940 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 3940 active.

How do I fix SPN 3940?

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 3940

J1939 SPN 3940 — Body/Chassis parameter

What it means

SPN 3940 typically indicates a 'Body/Chassis Parameter' error, often signifying that the ECM has received invalid data or is missing a required message from the Body Controller regarding chassis configuration or PTO (Power Take-Off) status.