What causes a U3421 code?

U3421 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 U3421 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with U3421 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

U3421

Manufacturer-specific network code U3421

What it means

Lost communication with the Trailer Interface Module. The truck's body controller is not receiving confirmation that the trailer lights/brakes are connected properly.