Android phones equipped with a proximity sensor disable the screen when you hold the phone close to your face while you are on a call. The point is to save power -- since you cannot use the phone's touchscreen for checking Facebook when you are pretending to pay attention to your phone call. In the event that your phone's screen will not shut off, or it shuts off all the time, you may be correct to blame the proximity sensor. Before disabling your proximity sensor, however, the best troubleshooting step is to first clean your phone.
Clean Your Phone
Turn off your phone and blow into the speaker with compressed air.
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Ensure there is no debris or dust in the speaker of your phone. If necessary, use a toothpick or other implement to carefully remove debris.
Restart your phone and test the proximity sensor functionality of your phone. It has been noted that dust and debris in this area of your phone is the common culprit for a malfunctioning proximity sensor. The proximity sensor is located at the top of your phone and if you hold the screen at a slight angle, you can see the sensors. They look like small holes covered by the screen glass and are close to the speaker.
Disable the Sensor
Tap the "Phone" icon on your phone to open the Phone app. Then tap the "menu" button and select "Settings" or "Call Settings."
Disable the proximity sensor setting in this menu. The procedure and tools may vary depending on the model of phone you own and which version of Android you have. Older versions of Android may not allow the same access to hardware functions as newer firmware versions.
Retest your phone during a call. If you have disabled the proximity sensor, the screen should not turn off when you hold it close to your face or if you place your hand close to the screen during a call.