Have you done the CIT test? If not you can try
1)Tapping "Kernel Version" under About Phone until you launch CIT
2)Go to no.12 Light Sensor and see the value for "LightSensor" eg. 80.0lux. Then cover the black bar on top of your phone and see if the value decreases. If the value doesn't change then something might be wrong with the hardware inside.
I used he app Android tester...there is a change in Lux when sensor is covered. Somehow, I just don't see the screen brightness dimming when the sensor is covered. Does you screen dims when the sensor is covered?

