What causes a SPN 1638 code?

SPN 1638 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 1638 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 1638 active.

How do I fix SPN 1638?

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 1638

High Hydraulic Retarder Oil Temperature

What it means

Indicates that the transmission or auxiliary hydraulic retarder fluid has exceeded the normal operating temperature threshold. This typically triggers a dash light and can lead to thermal protection derates.