What causes a SPN 3978 code?

SPN 3978 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 3978 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 3978 active.

How do I fix SPN 3978?

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 3978

J1939 SPN 3978 — Body/Chassis parameter

What it means

Power Take Off (PTO) Drive Disengage Activation - Abnormal Frequency, Pulse Width, or Period. This code typically pertains to the communication or electrical integrity of the PTO engagement system via the J1939 data link or body-to-engine interface.