Thanks to all who contributed. I had similar issue with the proximity sensor on iPhone 6s. The ear piece heats up and sensor fails to switch off the screen . I replaced the complete screen from another iPhone 6s with has faulty a board and it's works perfectly. I tried interchanging the sensors. The sensor fails to work on the new screen . The sensor works/screen goes off when I placed my finger on top of the screen but not on the sensor.
Most at times Sensor is not the problem.
Problem : Screen ( most of the screens are the cause)
Solution: Quality Screen / New Sensor