What should I check on a second-hand laptop or phone before buying?

A used device hides its problems well. Photographs show the case and nothing else, and the faults that matter most — a worn battery, an account lock, a hinge starting to crack, a port that only works at an angle — are all invisible until someone switches it on and works through them deliberately.

What you'll need

• The device switched on and charged
• A charger, to test charging
• A USB stick or a phone with a cable, to test ports (optional)
• A quiet twenty minutes with the seller

Step by step

1. Check for an activation or account lock before anything else.Have the seller reset the device in front of you, or confirm it starts to a fresh setup screen without asking for anyone's account. A device still tied to a previous owner cannot be used, sold or often even repaired, and there is no legitimate way around it.
2. Look up the model and its support window.Find out how much longer it receives security updates. This is what ends the useful life of most phones and tablets and increasingly of laptops, and it should drive the price you are prepared to pay.
3. Read the battery health and cycle count.Phones report a health percentage. Laptops report design capacity against current capacity, and a cycle count, through the system information or the manufacturer's utility. A high cycle count on a young device tells you it has been worked hard.
4. Display a full-screen white image, then a black one.White reveals dead pixels, dark blotches and pressure damage. Black reveals stuck bright pixels, backlight bleed and uneven lighting. Nothing else finds these, and a busy desktop background hides them completely.
5. Open and close the lid slowly, twice.Watch for the picture changing, for the hinge feeling gritty or uneven, and for the case flexing or creaking near the hinge. Hinge failures on laptops crack the plastic around them and are an expensive repair.
6. Test every port with something real.Plug a stick into each USB socket, a cable into the charging port, and a lead into the video output if you have a screen to hand. Wiggle each connection gently — a socket that works only at an angle is a repair waiting to happen.
7. Type across the whole keyboard and use the trackpad.Open a text document and press every key, including the function row. Check the trackpad clicks evenly at all four corners; uneven clicking is the classic first sign of a swelling battery underneath.
8. Check the case sits flat and the seams are closed.Put it on a flat surface and see whether it rocks. Look along the seams for a gap. Both are signs of a swollen battery, which is a safety issue rather than a bargaining point.
9. Test wifi, sound, camera and microphone.Connect to a network, play something with sound, and open the camera app. These take two minutes and each is a common fault that is invisible otherwise.
10. Let it run on battery while you finish the checks.Twenty minutes of ordinary use with a note of the percentage at the start and end is worth more than any reported figure. A device that drops twenty per cent in that time has a problem.
11. Meet somewhere sensible and pay in a way you can trace.A public place with power sockets, in daylight. Avoid payment methods with no recourse. If the device is stolen you lose both the money and the device, and a serial number that the seller will not let you check is a reason to walk away.

Tips

• Ask for the serial or IMEI number in advance and check it against the manufacturer's warranty lookup. It confirms the model, the age, and whether any warranty remains.
• A laptop that has clearly lived on a desk is usually a better buy than a slim one that has lived in a bag. Look at the corners and the port surrounds for the story.
• If the seller will not let you switch it on, that is the answer. There is no acceptable reason for it.

Common mistakes

• Buying without seeing it start to a fresh setup screen — An activation lock is the one fault that makes the device worth nothing, and it is invisible on a machine that is already logged in.
• Judging the screen against a desktop wallpaper — A patterned background hides dead pixels, backlight bleed and pressure marks entirely. Plain white and plain black at full screen take thirty seconds and find all of them.

If it doesn't work

Questions people ask