220-1101 Mobile Devices Practice Question
A user reports that their smartphone's touch screen stopped working after the phone was briefly submerged in water. The technician dried the device thoroughly, then replaced the screen assembly (including the digitizer) with a known-good part. After reassembly, the touch screen is still completely unresponsive. The display works correctly, and the phone powers on and charges normally. Which of the following components should the technician check next?
⚠ Common exam trap
Many candidates assume replacing the screen assembly always fixes touch issues, but water damage often travels via the flex cable to the motherboard's controller IC, which is a separate component not replaced with the screen.
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
✓
The motherboard's touch screen controller IC
Water damage can short or corrode the touch screen controller IC on the motherboard, which processes signals from the digitizer. Since the display and other functions work, the issue is isolated to the touch input path, and replacing the screen assembly (including the digitizer) did not resolve it, pointing to a damaged controller IC rather than the peripheral components.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The battery
Why it's wrong here
A faulty battery can certainly cause boot loops, random shutdowns, or charging failures, but it cannot selectively disable the touch digitizer. The smartphone powers on and charges normally, which proves the battery is delivering adequate DC power to the PMIC and downstream regulators. Touch input relies on digitizer supply rails and the touch controller IC, not on battery health alone; a failing battery would produce system-wide power symptoms, not an isolated loss of touch responsiveness.
- ✗
The charging port
Why it's wrong here
The charging port is electrically coupled to the USB controller and PMIC for battery charging and data transfer, while the touch digitizer communicates over a separate internal I2C or SPI bus to the motherboard's touch controller IC. Physical damage to the charging port would typically manifest as no charging, no USB data, or an 'accessory connected' error, not as a dead touch screen. Because these signal paths are entirely distinct, a defective port cannot be the root cause of a non-responsive touch screen.
- ✓
The motherboard's touch screen controller IC
Why this is correct
The motherboard's touch screen controller IC is the SoC-side chip that drives the digitizer's analog lines, measures capacitance changes, and converts them to digital touch coordinates. Water damage can corrode or short this IC's BGA solder balls or internal traces, causing the screen to remain fully lit while producing zero touch input—even if a brand-new digitizer is installed. The failure persists after screen replacement because the problem lies on the motherboard, not the display/touch assembly.
- ✗
The SIM card slot
Why it's wrong here
The SIM card slot connects only to the baseband/modem processor through the SIM interface for subscriber authentication and network registration. It shares no data lines, power planes, or buses with the touch screen controller or digitizer. Even a severely damaged SIM slot would generate SIM-related errors (e.g., 'No SIM' or 'SIM not detected') while leaving touch input completely unaffected; thus it cannot explain a non-working touch screen.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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