My Sonicare toothbrush turns off by itself

The selected brushing mode defines the brushing time. Your toothbrush will turn off automatically when the recommended time is up. 
We recommend changing the brushing mode. Brushing mode times can range from 20 seconds to 3.5 minutes. 

The BrushPacer divides the brushing time into six parts. While brushing, it briefly stops vibrating to indicate when to move to the next area. The toothbrush automatically stops at the end of the brushing session.

Your toothbrush measures the pressure you apply while brushing. If you press too hard, it briefly stops, indicating that you are using too much pressure.

If none of these tips help, your toothbrush may be damaged internally. We recommend that you request a repair or exchange for your toothbrush.