What causes a P0621 code?

P0621 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 P0621 code?

Drivability depends on the severity. Check the severity rating and verified driver reports on this page before continuing to drive with P0621 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

P0621

Alternator Lamp Control Circuit Malfunction

What it means

The Engine Control Module (ECM) has detected an unexpected voltage signal or open circuit on the alternator indicator light control circuit. This indicates a failure in the communication link between the ECM and the charging system monitor.