What happens when a voltage regulator fails?

When a voltage regulator fails, it primarily impacts the vehicle's electrical system, particularly affecting the charging of the battery. The voltage regulator is responsible for maintaining a consistent voltage level to ensure that the battery is charged correctly while the engine is running. If the regulator fails, it can cause overcharging or undercharging of the battery.

In the case of overcharging, the battery may get damaged, leading to reduced battery lifespan or even failure. Conversely, undercharging means that the battery might not receive enough power, leaving it unable to start the engine or operate electrical components effectively.

The other choices suggest broader operational issues within the vehicle, such as stalling frequently or affecting the fuel system. These issues can occur due to electrical problems caused by a failing voltage regulator, but they are not direct consequences. The most immediate and clear effect of a failing voltage regulator is indeed the inability of the battery to charge properly.