Why Your Phone Screen Stays Dark During Calls: Proximity Sensor Issue
A user reports that their smartphone's screen does not turn off during phone calls when held to the ear, causing unintended button presses. The screen works normally at all other times. The technician has verified the device's operating system and apps are up to date. Which of the following is the MOST likely cause of this issue?
⚠ Common exam trap
CompTIA often tests the distinction between the proximity sensor and the ambient light sensor, as candidates may confuse their functions—both use light but for entirely different purposes (screen-off vs. brightness adjustment).
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
A faulty proximity sensor
The proximity sensor, typically an infrared (IR) LED and photodetector, detects when the phone is held to the ear during a call and disables the touchscreen to prevent unintended inputs. Since the screen works normally at other times and the OS/apps are updated, a faulty proximity sensor is the most likely cause, as it fails to trigger the screen-off command during calls.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
A faulty ambient light sensor
Why it's wrong here
The ambient light sensor (ALS) measures surrounding illuminance to adjust the display backlight; it has no role in the call-state logic or the kernel's sleep/wake decisions. Even if the ALS is defective and reports a very low light level, the screen would simply become dimmer or go to minimum brightness — it would never remain fully lit during a call. Screen-off during calls is governed exclusively by the proximity sensor's near/far state.
- ✓
A faulty proximity sensor
Why this is correct
The proximity sensor emits infrared light and measures reflection to determine if the device is held close to the user's ear. When the sensor reports near, the operating system immediately turns off the display to prevent accidental cheek taps and to save power; if the sensor is faulty and never reports near, the display stays on for the entire call. This is the classic symptom of a failed or blocked proximity sensor, and it is isolateable by temporarily covering the sensor window.
- ✗
Incorrect display calibration
Why it's wrong here
Display calibration defines the mapping between touch input coordinates and displayed pixels, plus color/gamma responses. It does not interact with the power management subsystem that controls the display sleep state during calls. A miscalibrated display produces ghost touches, offset taps, or washed-out colors at all times, not a persistent screen-on condition specifically during a call. Thus, recalibrating the display would not affect this behavior at all.
- ✗
A damaged digitizer
Why it's wrong here
A damaged digitizer — the touch-sensing capacitive layer — primarily causes touch-related defects such as phantom touches, unresponsive zones, or consistently erratic input, regardless of whether a call is active. The digitizer is not responsible for the display's on/off control; the display sleep/wake operation during calls is handled by the proximity sensor and the application processor. Consequently, a broken digitizer would show symptoms everywhere, not just in the call screen, and would not keep the screen from turning off when held to the ear.
Go deeper
Related to this question
Learn chapter
Mobile Operating Systems: iOS and Android
Key term
Light-emitting Diode
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current passes through it, commonly used as an indicator or illumination source in electronic devices.
Key term
Infrared
Infrared (IR) is a type of wireless communication that uses light waves just below visible red light to send data or control devices over short distances.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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