Re: Xperia SP Switches off automatically
Welcome to Android Central! That could mean a failing battery. Since the battery is not user-removable, you'd have to bring it to a Sony service center or an independent repair shop to get it replaced.
Also try booting into Safe Mode, which prevents all 3rd party apps from opening initially. If the problem goes away, then something you installed might be making the phone unstable. You can open apps one by one in Safe Mode to determine which one might be causing problems. To get into Safe Mode, while the device is on, press and hold Power to bring up the power menu. Then press and hold the Power Off option until you see the Safe Mode prompt. Tap OK, and it will boot into Safe Mode. To get out of Safe Mode, reboot normally.