What causes a SPN 3974 code?

SPN 3974 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 3974 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 3974 active.

How do I fix SPN 3974?

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 3974

J1939 SPN 3974 — Body/Chassis parameter

What it means

The engine or vehicle control module has detected a configuration or parameter error related to the body/chassis integration (e.g., PTO settings, neutral safety interlocks, or weight/axle configurations).