What causes a SPN 977 code?

SPN 977 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 977 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 977 active.

How do I fix SPN 977?

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 977

SPN 977 Fan Clutch Switch State Signal Malfunction

What it means

This fault indicates an electrical circuit failure, short, or unexpected state in the cooling fan clutch control circuit, preventing proper engine cooling management.