Samsung has some secret codes for your Galaxy S4 and other Galaxy smartphones too, for their service center employees to find easy way work round.
With these internal function (secret codes) of Galaxy S4 you can find out if your smartphone is working properly or not. You can test your LCD, Vibration, Camera, Sensors (accelerometer sensor, proximity sensor, magnetic sensor), touch screen, speaker, sub key etc.
To test, you can do following steps :
Open your Keypad
Then dial the secret code *#0*#.
Now you get a screen with title LCD TEST and below that you have lots of option to test various hardware parts of your phone such as speaker, sensor, lcd, etc
To go back use right physical button (back button), while testing the touch, TSP Hovering you have to touch and mark all the squares back button does not work.
Other useful secret codes for android phones are provided below :-
* #1234# to check software version of phone.
*#12580*369# to check software and hardware information.
*#0228# Battery status (ADC, RSSI reading)
*#0011# Service Menu