How to check a used phone before you buy it
Most bad used-phone deals fall into four traps: a copy dressed up as a famous model, a phone locked to someone else's account, a battery that is nearly worn out, and a part that fails only in one corner of the screen or one speaker. Each of them can be caught in a few minutes if you know where to look.
Before you meet
1. Look the model up
Write down the real processor, memory and storage options, screen size and number of cameras of the exact model. Copies often show the right name in Settings but the wrong processor, much smaller storage or fewer cameras. Ask the seller for the IMEI number and for photos of the box.
Meet in daylight, in a public place, with a charger cable and your own SIM card.
Locks and paperwork
2. Match the IMEI
Open the phone app and dial *#06#. The IMEI it shows must match the box and, on many phones, the SIM tray. Then ask your network operator to check that the IMEI is registered and not blocked (in Ethiopia, Ethio Telecom or Safaricom).
3. Make sure no account locks it
A phone still signed in to the seller's Google account will ask for that account after a reset, and you cannot use it. Ask the seller to remove their Google account (Settings, Accounts) and the maker's account (for example Samsung account), then reset the phone in front of you. Walk away from a phone that shows a company management message or a locked bootloader warning at start-up that the seller cannot explain.
Is it really that model?
4. Check the hardware, not the label
The name in Settings, About phone, is just text that anyone can change. Compare what the phone really has: the processor's core layout and graphics chip, the usable memory and the real storage size. Fake storage is common in copies: the phone reports 256 GB but data written past a few gigabytes is lost. Only writing data and reading it back proves the size.
Knowfone does this automatically: it compares the processor, graphics, memory, screen, storage and cameras with the model the phone claims to be, writes and reads back real data, and asks the security chip for a signed answer checked up to Google's root.
By brand: Tecno and Infinix, Samsung Galaxy, and test codes for every brand.
The body and the screen
5. Inspect the body
Look along the edges in good light. A screen or back that lifts, a gap that grows when you press, or a warm bulge points to a swollen battery: do not buy it. Damaged screw heads, uneven glue and a screen that sits higher on one side suggest the phone was opened. Check the camera glass for cracks and the charging port for dirt or green corrosion.
6. Test the screen
Look at full-screen white, black, red, green and blue pages at full brightness. Look for dead or stuck pixels, lines, a yellow or pink tint, bright patches near the edges, and the ghost of the keyboard or status bar on grey (burn-in, common on older OLED screens).
7. Test touch everywhere
Drag one finger slowly over the whole screen, including the edges and corners. A replaced or damaged screen often has a strip that does not respond, or that jumps. Try two and three fingers at once.
Battery and performance
8. Check the battery
Some phones show battery health in Settings, others in the maker's support app (for example the diagnostics in Samsung Members). Below about 80 percent of the original capacity a battery holds noticeably less. Watch the charge for a few minutes of heavy use: a sudden drop of several percent, or a phone that turns off above zero, means the battery is worn. Also check that the phone does not get hot while idle.
Cameras, sound, buttons, radios
9. Try every camera and the flash
Switch through every lens, front and back. Focus on something close and something far, film ten seconds of video and turn the flash on. Blurry spots that move with the phone, a lens that never focuses, or a camera that does not open are repairs.
10. Test speakers and microphones
Play music and cover each speaker in turn. Record a voice memo and listen back. Make a short call to check the earpiece and that the other person hears you.
11. Test buttons, the port and vibration
Press volume up, volume down and power. Plug in your own cable: the phone should show charging at once and keep charging when you move the cable gently. Turn on vibration and feel that it is strong and even. Try the fingerprint reader.
12. Test SIM, Wi-Fi, Bluetooth and GPS
Put in your own SIM and make a call and use mobile data. Join Wi-Fi, turn on Bluetooth and see that it finds devices, and open a map to see that location works.
Software and price
13. Check the software
Note the Android version and the date of the last security update. A modified (rooted) phone, or one with an unlocked bootloader, can be made to report anything about itself, which makes every other check less reliable.
14. Agree a fair price, or walk away
Every problem has a repair cost. A worn battery, a dead strip on the screen or a weak speaker is a reason to pay less. A copy, a locked account, a swollen battery or fake storage is a reason to walk away.
What no one can check on the spot
No app and no quick look can show past water damage, internal repairs, or exactly how many months a phone will last. Ask for the receipt, the box and the reason for selling. Buying from a shop that gives a warranty lowers that risk.
Do most of this in about 7 minutes
Knowfone runs the checks in this list that a phone can test on itself: identity and storage, security, every part of the touch screen, colour pages, speakers and microphones, cameras and flash, battery, processor, sensors, buttons, port, vibration and radios. Then it tells you plainly: good to buy, buy only at a lower price, or don't buy, with the reasons. It is free.
Many of these checks run in the phone's browser too: the free online phone test. Real specs of popular models: is my phone original?