What causes a SPN 4509 code?

SPN 4509 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 4509 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4509 active.

How do I fix SPN 4509?

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 4509

Allison transmission code P4509

What it means

Transmission Output Speed Sensor Circuit Low Signal or Intermittent fault. The TCM is receiving an implausible or missing signal from the output speed sensor.