What causes a SPN 520979 code?

SPN 520979 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 520979 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 520979 active.

How do I fix SPN 520979?

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 520979

Clutch Engagement Slippage

What it means

This code indicates that the transmission is detecting excessive clutch slippage during engagement. This can lead to premature wear and overheating of the clutch.