Touch Test

What touch test does

A touchscreen test paints wherever it detects contact, so anywhere that stays blank is a region the digitiser is not registering. It also counts how many simultaneous touches the screen reports, which is the other thing that commonly goes wrong. Both faults are usual after a screen replacement or a drop, and both are worth checking while a repair is still under warranty or a device is still returnable.

How to use it

1. Go full screen so the test covers the whole display rather than part of a page.
2. Drag one finger slowly across every part of the surface, right into the corners.
3. Watch for any patch that stays unpainted. That is a dead zone.
4. Put as many fingers down at once as you can and compare the count with what your device supports.

About touch test

How a touchscreen actually senses a finger

Nearly every modern touchscreen is projected capacitive. A grid of transparent electrodes sits under the glass holding a small electrical charge, and a finger, being conductive and mostly water, changes the capacitance where it touches. The controller reads which intersections changed and by how much, and interpolates a position more precise than the grid spacing. That is why a gloved finger usually does nothing, why a stylus has to be conductive or actively powered, and why water on the screen produces phantom touches: the controller cannot distinguish a puddle from a fingertip.

What a dead zone means

A region that registers nothing usually means a broken connection between the digitiser layer and its controller, most often at the flex cable, which is the part that gets disturbed during a repair or stressed by a drop. Dead zones are typically bands or strips rather than round patches, because a single trace serves a whole row or column. If a strip along one edge has stopped working, that is the classic signature, and it is a hardware fault rather than anything software can repair.

Multi-touch limits, and what they tell you

Most phones report five or ten simultaneous touches and most tablets ten. A screen that reports fewer than the model is specified for usually has a replacement digitiser of lower quality than the original, which is common in third-party repairs and worth knowing about before the warranty on the repair expires. This page shows both what your device claims to support and what it actually registered, and a gap between the two is the interesting result.

Phantom touches and how to tell them apart

If marks appear without you touching anything, that is a phantom touch, and it has three common causes. Moisture on the surface is the easiest to rule out: dry the screen and try again. A poor quality or badly fitted screen protector, particularly one with trapped air or moisture underneath, is the next. A failing digitiser or a charger producing electrical noise is the third; if it only happens while charging, try a different charger before blaming the screen.

Before you conclude the hardware is broken

Restart the device first, since a stuck touch driver is a real and easily fixed cause. Remove any case and screen protector and test again. Clean the screen, because a heavy layer of grease genuinely degrades sensitivity. And test in your device's own safe mode if it has one, which rules out an app interfering with touch input. If the fault survives all of that and appears in the same place every time, it is hardware.