What causes a SPN 638 code?

SPN 638 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 638 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 638 active.

How do I fix SPN 638?

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 638

J1939 SPN 638 — Engine Management parameter

What it means

Rack Position Actuator - This fault indicates the Engine Control Unit (ECU) has detected a deviation between the commanded fuel rack position and the actual position signal returned by the sensor.