What causes a SPN 4603 code?

SPN 4603 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 4603 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4603 active.

How do I fix SPN 4603?

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 4603

Allison transmission code P4603

What it means

Internal Diagnostic Loopback Check Failure - This fault indicates a self-test failure within the TCM monitoring its internal circuits or communication loops.