Proximity sensor is not working

In the beginning, when the proximity sensor didn't work, I got it to work by rebooting the phone. A few days later it stopped working again, and when I tried to test it with the "Sensors Test" application - the application saw all the sensors except the proximity sensor, as if it had never been on the phone.

I reset the phone to factory settings but nothing helped. Please let me know if there is a way to fix the problem. If the problem is in the sensor itself - please tell me its code so that I can find where to order it for replacement.