What causes a U3391 code?

U3391 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 U3391 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with U3391 active.

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

OBD-II

U3391

Manufacturer-specific network code U3391

What it means

Manufacturer network error involving the aftertreatment or emission control digital link. Commonly seen in US10 and newer diesel systems.