What causes a U0821 code?

U0821 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 U0821 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with U0821 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

U0821

Network/CAN bus communication code U0821

What it means

Communication failure with the Radar/Object Detection Module. This impacts Advanced Driver Assistance Systems (ADAS) like collision mitigation.