What causes a SPN 978 code?

SPN 978 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 978 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 978 active.

How do I fix SPN 978?

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 978

J1939 SPN 978 — Engine Management parameter

What it means

SPN 978 refers to the Fan Drive / Air Conditioning Clutch output circuit. It indicates an electrical fault (short to ground, short to battery, or open circuit) in the A/C compressor clutch control circuit.