What causes a SPN 4304 code?

SPN 4304 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 4304 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4304 active.

How do I fix SPN 4304?

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 4304

Allison transmission code P4304

What it means

Transmission Output Shaft Over-Speed or Gear Ratio Error during specific shift transitions. Indicates the transmission sensed an incorrect ratio, usually implying internal slipping.