* Alternator failure: This is the most likely culprit. The alternator is responsible for charging the battery while the engine is running. If it's failing, it can't produce enough voltage, causing the light to illuminate.
* Alternator belt broken or slipping: Even a good alternator won't charge the battery if the belt driving it is broken or slipping.
* Battery problems: A failing battery might not be able to hold a charge, even if the alternator is working correctly. This could cause the voltage light to come on.
* Wiring problems: A loose connection, corroded wire, or a broken wire in the charging system circuitry can prevent the voltage from reaching the battery or the warning light system.
* Voltage regulator malfunction (in some cases): Older vehicles might have a separate voltage regulator; a faulty one can also lead to charging problems.
To diagnose the issue, you'd need to check these components. A multimeter is very helpful for testing the alternator's output and the battery's voltage. A visual inspection of the belt and wiring is also recommended.
