What are the common causes of P0621?

An open or short in the L terminal wire. Corrosion at the L terminal connector. A failed alternator. A failed charging indicator lamp. A faulty ECM output.

Check the charging indicator lamp and its circuit. Inspect the L terminal wire and connector. Measure voltage on the L terminal with the key on. Test the alternator charging output. Repair or replace the L terminal circuit if the lamp is dead. Clear the code and retest.

P0621

Generator Lamp/L Terminal Circuit

System: Computer output circuits (P code)
Lamp: Check Engine Light
Category: Powertrain - Computer and Auxiliary Controls

What it means

P0621 means Generator Lamp/L Terminal Circuit.

Check these first

An open or short in the L terminal wire
Corrosion at the L terminal connector
A failed alternator
A failed charging indicator lamp
A faulty ECM output

What to do next

Check the charging indicator lamp and its circuit
Inspect the L terminal wire and connector
Measure voltage on the L terminal with the key on
Test the alternator charging output
Repair or replace the L terminal circuit if the lamp is dead
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.