Screen and microphone test
Check a new or second-hand phone, laptop or screen in a minute: hunt for dead or stuck pixels, then make sure the microphone hears you.
Screen: dead or stuck pixels
The whole screen turns black, white, red, green and blue in turn. Look closely at each colour for a dot that doesn't match. Turn the brightness up and wipe the screen first, so dust doesn't fool you.
Tap for the next colour. Tap and hold, or press Esc, to stop at any time.
Microphone
Your browser asks for permission only when you tap the button. Talk, clap or blow gently: the bar should jump.
Camera (optional)
Shows what your camera sees, like a mirror. Nothing is recorded or saved.
Runs on your device. Nothing you see, say or record is uploaded or saved: a recording lives only in this page and is gone when you leave.
What to look for
A dark dot that stays black on every colour. Easiest to spot on white.
A dot stuck on one colour: red, green or blue. Easiest to spot on black, or on a colour that doesn't match it.
A white dot that stays lit. You see it on black.
A dot that moves when you wipe is dust. Patches, lines or uneven glow are a different problem: show the seller.
Run the test in the shop or before you hand over the money. Makers set their own rules on how many faulty pixels they accept, so ask about the return or warranty policy before you buy.
Check that the browser has permission (the lock or settings icon next to the address), that no case or finger covers the small mic hole, and that another app isn't using it.
Clean the mic hole gently with a soft brush. If calls still sound bad, try a wired or wireless headset to see if the built-in mic is the problem.
Sources: the international display standard ISO 9241-307 groups pixel faults into types (always lit, always dark, and partly lit sub-pixels) and fault classes; manufacturers decide how many they accept. The microphone level is measured in your browser with the Web Audio API (W3C), in decibels relative to the loudest sound the mic can record (0 dB). Each pixel of a colour screen is made of red, green and blue sub-pixels, which is why a stuck one shows as one of these colours. This page does not repair pixels and is not a diagnosis.