What causes a SPN 4338 code?

SPN 4338 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 4338 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with SPN 4338 active.

How do I fix SPN 4338?

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 4338

Detroit DD13/DD15/DD16 SPN 4338

What it means

SPN 4338 indicates that the Aftertreatment 1 SCR DEF Pressure is incorrect (often specifically referring to the pressure being out of range or failing to reach the commanded setpoint).