What causes a SPN 4908 code?

SPN 4908 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 4908 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4908 active.

How do I fix SPN 4908?

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 4908

Allison transmission code P4908

What it means

P4908 indicates a 'Main Shaft Speed Sensor Circuit Performance' or 'Output Speed Sensor / Input Speed Sensor Correlation' error where the TCM detects a mismatch between calculated and actual shaft speeds.