If the timer on a microwave has reached zero, but the light and fan are still running, it generally means that the microwave is still operational, but the cooking process has completed. The light and fan may continue running to provide illumination and ventilation while you remove the food from the microwave and allow it to cool.
In most microwaves, the timer and cooking functions are separate. The timer controls the duration of the cooking process and alerts you when the set time has elapsed. Once the timer reaches zero, it indicates that the specified cooking time has finished, and the microwave stops actively cooking the food.
However, the light and fan can remain on for various reasons, such as for safety, ventilation, or to indicate that the microwave is still powered on. They don't necessarily indicate that the microwave is still actively cooking the food.
To confirm if the cooking process is still ongoing, you can open the microwave door and check if the heating elements (usually the microwave generator and magnetron) are still active. If the microwave is no longer generating heat, it means the cooking process has completed, and you can safely remove your food.