Dead Pixel: checking a screen for bad pixels, touch dead zones and a weak vibration motor

Dead Pixel is a screen check: pick a test from the menu, look or touch, then go back and pick the next. It's useful before you buy or sell a second-hand phone, after a screen repair, or when something on the display just looks off. You can also run it in your browser, which is a quick way to check a laptop screen.

Before you start

Wipe the screen. A speck of dust looks exactly like a dark pixel.
Turn brightness up and look at each pattern straight on, then from a slight angle.

The five pattern tests

These fill the whole screen. The control bar stays hidden for three seconds so nothing covers the pattern, then it comes back with Back, the step number and a button that moves the bar to the top or bottom, so you can also see the strip it was covering. Tap anywhere to go to the next step; after the last step you're back in the menu.

Solid Colors: ten fills in a row: black, white, red, green, blue, cyan, magenta, yellow, mid grey and dark grey. A dot that stays lit on black is a stuck pixel; a dot that stays dark on white is a dead one. A dot that looks wrong only on red, green or blue usually means one colour of that pixel is stuck. The two greys are best for spotting uneven backlight and patches of tint.
RGB Bars: red, green, blue, white and black stripes side by side. A stripe that looks dim or tinted compared with the others points to a problem with that colour channel.
Fine Grid: black lines on white, one or two device pixels apart. A single odd dot breaks the regular pattern and is much easier to see here than on a photo.
Checkerboard: black and white squares of the same tiny size, alternating. Stuck dots stand out as a break in the pattern.
Gradient: three steps (left to right, top to bottom, corner to corner) running from black through red, green and blue to white. Look for visible bands where the colour should flow smoothly, and for areas that look tinted.

Seven touch and pen tests

Here the bar stays visible, and most tests have a Clear button to start over.

Touch Test: the screen is split into cells about a fingertip wide. Tap every one; each turns green, and when all are done you see "All cells OK!". A cell that never lights up is a dead zone.
Touch Move: drag to draw. Draw a few slow straight lines: gaps or zigzags mean the screen loses track of your finger.
Hover: move a pen or mouse over the screen without pressing, and a short trail follows it. Fingers don't hover, so this test only shows something with a device that reports hover.
Multi-Touch: put two to five fingers down. Every contact should show its own dot; count how many the screen tracks at once.
Edge Sweep: drag along all four edges, close to the border. An edge counts once your finger gets about two thirds of the way along it, and when all four are done you see "All edges OK!". Edges are where cracked or badly refitted screens fail first.
Pinch: pinch and spread with two fingers. The distance between them is shown in pixels and should change smoothly, without jumps.
Pressure: draw with a stylus. A ring and a percentage show how hard you press. If it stays at 50% whatever you do, the device isn't reporting pressure at all, which is normal for fingers.

Vibration

Two buttons: one short pulse and a short pattern. A weak, rattling or missing buzz is worth noticing on a used phone. If the device can't vibrate, the screen says "Vibration not supported".

Found something?

Run the same test again after a few minutes: some stuck pixels come and go. Take a photo of the dot on a black and on a white fill and note where it is on the screen. That's what a seller or repair shop will want to see.

More on the Dead Pixel page, or open the tests.