A P0621 code in your 2021 Acura MDX indicates an issue with the generator lamp control circuit—the circuit that controls your charging system warning light on the dashboard. This code typically means the ECM has detected a problem with how the charging indicator lamp is being controlled or that the lamp feedback isn't matching expectations.
How the Generator Lamp Circuit Works
The charging system warning light does more than just indicate problems. In many vehicles, including your MDX, the generator lamp circuit provides feedback to the ECM about alternator status. The lamp receives power through the alternator's field circuit; when charging begins, the lamp should turn off. P0621 indicates this circuit isn't operating as expected.
Common Causes
A burned-out indicator bulb (if using a traditional bulb) is the simplest cause—the ECM detects no lamp current and flags an issue. LED indicator malfunctions in modern instrument clusters can trigger this code. Wiring issues between the instrument cluster, ECM, and alternator affect the circuit. Alternator internal issues that prevent proper lamp circuit operation also cause P0621.
Symptoms You May Notice
The charging system warning light may be always on, always off (even during key-on), or behaving erratically. The check engine light illuminates with P0621 stored. Actual charging system function may be normal despite the warning light malfunction.
Diagnostic Procedure
Using Honda HDS or a compatible scan tool, monitor generator lamp circuit data. Check if the charging indicator responds properly during key-on (should illuminate) and after engine start (should extinguish). Test the circuit for proper voltage levels at various states. If using LED instrumentation, the cluster module may need diagnosis for internal lamp driver issues.