Testing a second-hand phone or tablet before you pay

Test a second-hand phone or tablet in front of the seller, before any money changes hands. Check the touchscreen for dead areas and ghost touches, the screen for dead pixels and burn-in, both cameras, the microphones, the speakers and earpiece, the buttons and the charging port. Then check that the device is not still tied to the seller's account, which no browser test can show. Each check takes a minute or two, and the tests run in the phone's own browser, so nothing needs installing.

Touchscreen: dead zones and ghost touches

1. Open the touch screen test and choose Grid. Touch every square, one at a time, with a fingertip. A grey square was never touched. Test the grey squares again, and check the edges and corners, because a loose protector or a damaged touch layer can leave dead areas there.
2. Choose Draw and drag across the whole area with two or more fingers. The most-at-once figure shows how many touches the browser received together. Phones differ in how many touches they can track, and the test can only count what the browser passes on.
3. Check for ghost touches. Leave the screen untouched while the page is open. The touches-now figure should stay at 0. If it changes while nothing touches the glass, the panel is reporting touches nobody made. A cracked screen or a damaged protector can cause this, so remove the protector before you judge.
4. If a dead zone or ghost touch remains after cleaning and removing the protector, the panel is probably at fault. Repairs to a touch panel are often a whole screen replacement, so compare that cost with the price. Touchscreen dead zones and ghost touches explains how to tell a fault from a setting.

Screen colours, dead pixels and burn-in

1. Open the dead pixel test and start the full-screen test. Step through the solid colours. A dead pixel stays black on every colour, and a stuck pixel stays one colour. Both are easiest to see on the solid steps, so do not skip any of them.
2. Look at the grey and white steps for burn-in. Burn-in is a faint image, such as a status bar, an app icon or a keyboard shape, that stays visible on a plain colour. It is a particular risk on OLED screens, where a static image can wear the pixels unevenly over time.
3. Check the corners too. A cracked glass can show as a dark line or a pale spot under a solid colour, even when the touch still works.

Cameras and microphones

1. Start the webcam test and check the picture. If the page offers a camera list, switch between the front and back cameras. If the list shows only one camera, use the phone's own camera app to check the other. Look for dust, scratches and fogging under the lens, and check that the picture is sharp in good light.
2. Speak with the microphone test. Watch the level move, then record five seconds and play them back. The browser uses the microphone it chooses, which may not be the one you speak into. Record a short voice note with the phone's own recorder as well, and listen for hiss or a muffled sound. A phone has more than one microphone, and each can fail on its own.
3. Make a short call to a number you can reach, and hold the phone to your ear to test the earpiece. Check that the other person hears you clearly, and that you can hear them.

Speakers, buttons and charging

1. Play left, right and both in the speaker test with the volume low. Phones with stereo speakers play left and right from different places. A phone with one speaker still plays the tones, but you will not hear a left and right difference. Listen for crackle or distortion on the louder tones, and note any sound that is missing.
2. Press each physical button: volume up, volume down, power, and any side or home button. Each should respond, and the screen should wake or lock as expected. If the phone has a fingerprint sensor or face unlock, check that it responds. You can set it up with your own finger or face later.
3. Plug in the original charger, or a known good one, and check that the phone shows it is charging. Wiggle the cable gently, and look inside the port for lint. A charge that cuts out when the cable moves points to a loose port.
4. Check the battery's health figure. Recent iPhones show a maximum capacity figure in the battery settings. Many Android phones show a battery health reading too, though the menu depends on the maker. A low figure means the battery is worn, and a replacement may cost money.

Account locks

No browser test can show whether the phone is still tied to someone else's account, so check that before you pay.

• iPhone. Ask the seller to turn off Find My and sign out of their Apple account on the phone. Activation Lock stays on while Find My is on, and during setup it asks for the previous Apple ID and its password.
• Android. Ask the seller to remove their Google account from the phone before the sale. If that account stays on the phone, factory reset protection can ask for it after a reset.
• In person. If the seller can reset the phone in front of you, after backing up their data, watch the first setup screens. Stop if the phone asks for an account or password you do not know.