How to Check a Second-Hand Phone Before You Buy
A used phone hides its real condition behind a clean screen and a fresh factory reset. The four things that actually determine whether it is worth the money — battery health, screen defects, touch dead zones and failing sensors — can all be checked in the browser, on the phone, before you hand over anything.
Battery health is the number that decides the price
On an iPhone, this is not negotiable guesswork: open Settings → Battery → Battery Health & Charging and read Maximum Capacity. Apple treats 80% as the threshold at which a battery is considered consumed. Below that, expect a phone that needs charging twice a day.
Android is less consistent. Some manufacturers expose battery health in Settings, many do not. Where it is hidden, the practical test is behavioural: note the percentage, use the phone hard for ten minutes — camera, screen at full brightness — and look at the percentage again. A battery that drops several points in ten minutes of ordinary use is failing regardless of what any menu claims.
Run the battery test on the phone to confirm it reports a charge level and registers as charging when plugged in. A phone that will not report charging status, or that jumps erratically between percentages, has a battery or charging circuit problem.
Check whether the battery has already been replaced. On an iPhone, Settings → General → About will flag non-genuine parts. A replaced battery is not automatically bad, but it should be disclosed and it should affect the price.
The screen: look for what a photograph cannot show
Open the dead pixel test and cycle the solid colours full-screen with the brightness up.
- Black screen shows bright stuck pixels and, on an OLED, uneven patches where the panel has aged unevenly.
- White screen shows dark dead pixels, dust under the glass, and any yellowing.
- Red, green and blue isolate individual subpixels — a dot that disappears on one colour but not the others is a failed subpixel rather than a fully dead pixel.
On an OLED phone, burn-in matters more than dead pixels. Leave a solid grey or white field on screen and look for faint ghosts of the status bar, the keyboard, or navigation buttons. Those do not fade. If you can see them on a plain field, you will see them in daily use.
Touch dead zones: the fault people discover a week later
A cracked or previously replaced digitiser often works everywhere except one strip. You will not find it by swiping casually — you find it by covering the whole screen deliberately. The touchscreen test does exactly that: drag across every part of the display and watch for areas that do not register.
Test multi-touch too. A digitiser that handles one finger but drops the second will break games and pinch-to-zoom while seeming fine in a shop.
Sensors are where replaced phones give themselves away
Cheap screen repairs frequently damage or omit the proximity sensor and disturb the accelerometer. The symptoms arrive later: the screen does not blank during calls, auto-rotate stops working, the compass points the wrong way in maps.
The phone sensor test reads the accelerometer, gyroscope and magnetometer live in the browser. Tilt the phone and watch the values move. A sensor that reports nothing, or that reports a frozen value no matter how you move the device, is broken.
A drifting compass is often just uncalibrated rather than faulty — move the phone in a figure of eight and check again before counting it against the seller.
Camera, microphone and speakers
Open the camera test and check both the front camera and, if the browser lets you switch, the rear. Look for dust specks, haze and focus failure — a camera that hunts for focus and never settles usually has a damaged module.
Run the microphone test and speak at a normal volume. Then play tones through the speaker test. Phones have multiple microphones and often two speakers; one failing is common and rarely mentioned.
The checks that are not about hardware
- Verify the IMEI. Dial
*#06#to display it, then check it against the device's records and confirm it is not reported lost or stolen. A phone that is blacklisted by a carrier is worthless on that network. - Confirm the account lock is off. An iPhone still signed into someone else's iCloud, or an Android still tied to a Google account, cannot be used. The phone must be fully signed out in front of you.
- Check the charging port physically. Plug in and move the cable gently. A port that only charges at one angle needs replacing.
- Look at the water damage indicator where the model has an accessible one.
What is worth fixing
A worn battery on a common model is a routine replacement and a fair negotiating point. A cracked outer glass with a working digitiser is cosmetic. A failed sensor, a dead speaker, or OLED burn-in are repairs that cost a real fraction of the phone's value — and burn-in cannot be repaired at all, only replaced.
The deals that go wrong are rarely the obviously broken phones. They are the ones that looked perfect for the five minutes anyone bothered to look.
Sources: Apple — iPhone battery and performance · MDN — Sensor APIs
Frequently asked questions
Can a browser tell me a phone's battery health?
It can tell you the charge level and whether the phone is charging, but not the maximum capacity percentage. On an iPhone, read that in Settings, Battery, Battery Health and Charging. Many Android phones do not expose it at all, so use a ten-minute drain test instead.
What battery percentage is too low to buy?
Apple treats 80% maximum capacity as the point at which a battery is considered consumed. Below that on an iPhone, factor a battery replacement into the price. For Android without a health readout, judge by how fast it drains under real use.
How do I check for OLED burn-in?
Display a plain grey or white field full-screen at high brightness and look for faint outlines of the status bar, keyboard or navigation buttons. Burn-in does not fade, so anything visible on a plain field will be visible in use.
Why test the sensors on a used phone?
Screen repairs often damage the proximity sensor or disturb the accelerometer, and the symptoms only appear later — the display not blanking during calls, auto-rotate failing, or the compass pointing wrong. The sensor test shows live readings so you can check before buying.
What does checking the IMEI tell me?
Dialling *#06# shows the IMEI, which you can check against carrier blacklists. A phone reported lost or stolen can be blocked from the network, which makes it unusable regardless of its condition.
Do these tests need an app?
No. Everything here runs in the phone's browser, needs no installation and no account, and nothing you test is uploaded — which is also why a seller has no reason to refuse.