Buying a smartphone, especially on the secondary market or through dubious sites, always carries the risk of getting a faulty device or an outright Chinese fake. Even buying a new device from an untrusted store can result in trouble if you do not carry out basic diagnostics immediately after unpacking. Statistics show that about 30% of problems with Android devices that are brought to the service could be identified at the time of the transaction.
The verification procedure does not require expensive equipment or deep knowledge in electronics. All you need is a smartphone, Internet access and about 20-30 minutes of free time. In this article, we will analyze an algorithm of actions that will allow you to weed out refurbished devices, devices with matrix defects, “dead” batteries and phones listed as stolen.
Ignoring these steps may lead to you buying a gadget that will stop working in a week or will cost you more in repairs than the purchase itself. Therefore, approach the issue of choice seriously and methodically.
External inspection of the body and mechanical damage
The first thing diagnostics begins with is a visual inspection. Do not rush to turn on the device if you see obvious signs of being dropped or tampered with. Carefully examine the joints of the case and the screen frame. The presence of gaps, traces of glue or divergent seams often indicates poor quality repair after a serious impact or water exposure.
Pay special attention to the charging connector USB-C or Micro-USB. There should be no oxidation, dirt or play of the tongue inside it. If the connector is loose or the cable does not click into place, this is a sure sign of wear and tear that will require replacement in the near future.
Inspect the camera lenses. Lenses must be clean, without scratches or cracks. Even a microscopic crack on the protective glass of the camera will result in the pictures being cloudy or with reflections that cannot be removed by software.
⚠️ Attention: If spots resembling a rainbow or ink blots are visible under the protective glass of the screen, it means that the matrix has been severely compressed or broken. Over time, these spots will spread across the entire display.
Check the control buttons. The power and volume buttons should have a clear, elastic movement. If they fall through, stick, or are difficult to press, this may indicate moisture getting inside the case or mechanical damage to the cable.
Checking the authenticity and status of the device (IMEI)
Each legal smartphone has a unique identifier IMEI (International Mobile Equipment Identity). Verifying this number allows you to understand whether the phone is listed as stolen and whether it corresponds to the declared model. You can find the number in three ways: on the box, on the sticker under the battery (if it is removable) and in the device itself.
The fastest way to find out the IMEI on a switched-on phone is to enter the universal code in the “Phone” application. Dial the combination *#06#. One or two numbers will instantly appear on the screen (for dual-SIM models).
*#06#Compare the numbers that appear on the screen with those indicated on the box and in the warranty card. They must match until the last digit. If the numbers do not match, you have a device assembled from spare parts (“Frankenstein”), or a stolen device with the numbers mixed up.
Next you need to check the status of the device in the database. Enter the found number on specialized IMEI checking resources. This will show the device model, production date and, critically, the operator blocking status (blacklist).
What to do if the IMEI is not detected?
If, when you enter the code *#06#, nothing appears on the screen or “Null/Invalid” is displayed, this means that the software part of the phone is damaged or the device is a gross fake. It is strictly not recommended to buy such a device.
Diagnostics of the display and touch screen
The screen is the most expensive and fragile component of a smartphone. To test it in Android, there is a built-in engineering menu or special test applications. The most reliable method is to use an engineering code that triggers a hardware check mode.
The codes may differ for devices of different brands. For example, Xiaomi is used ##6484##, for Samsung — #0#. If the code does not work, download an application like TestM or Phone Check from the Google Play store.
In test mode, select the Display test item. You will be asked to swipe your finger across the colored squares. This is necessary to identify the “dead” zones of the sensor. Carefully monitor the reaction of the screen: if in some place the color of the square does not change when touched, it means that the touchscreen in this area is faulty.
- 🔍 Dead pixels: Turn on red, green, blue and white colors alternately. Carefully look for dots that differ in color from the general background or do not light up at all.
- 🌈 Color rendition: Evaluate the uniformity of the backlight. There should be no yellow spots or darkening around the edges on a white background (the effect of “stripes”).
- 🖐️ Multi-touch: Check support for simultaneous touches. Try tapping the screen with 5-10 fingers at a time. All points should be displayed correctly.
☑️ Checking the screen
Also test the proximity sensor. During a call, hold the phone to your ear and the screen should go dark. If this does not happen, you will press the buttons with your cheek during a conversation, which is extremely inconvenient.
Testing the camera, sound and microphones
Many buyers forget to check the audio path and cameras before signing the acceptance certificate. Launch the Camera app and take several pictures in different modes: daylight, portrait, macro. Pay attention to focusing - it should happen quickly and clearly.
Switch to the front camera and record a short video. This will help identify problems with stabilization and sound recording quality. If the main camera lens has optical stabilization (OIS), listen: a slight hum or rattling when shaking the phone may indicate a breakdown of the stabilization module.
To check the sound, open the voice recorder and record the speech, then play it back. After that, call someone or voicemail to check the main microphone is working during a call. Don't forget to also test the second microphone (noise reduction), which is usually located on the top end of the case.
⚠️ Attention: The speakers of modern smartphones are very sensitive to dust and moisture. If the sound wheezes at maximum volume even without music turned on, the speaker membrane may be damaged or clogged.
Check the operation of the vibration motor. In the sound settings, turn on vibration when touched and evaluate its intensity. Malfunctions in the vibration response often indicate problems with the power controller.
When checking the camera, be sure to wipe the lenses with a soft cloth before shooting. Often, buyers complain about “cloudy photos,” which are actually the result of dirty protective glass or factory film.
Assessing the condition of the battery and charging system
Battery condition is one of the most difficult parameters to assess without special equipment, since Android does not always show the exact percentage of wear in the standard menu. However, there are indirect signs that allow you to judge the health of the battery.
Connect an original or high-quality charger. The phone should start charging immediately, without any delay. During charging, the case should not heat up excessively. Slight heat in the processor area is acceptable, but a hot battery is a sign of degradation of chemical elements.
To obtain more accurate data, you can use applications like AccuBattery or Ampere. They will show the actual battery capacity in mAh and charging speed in milliamps. Compare the obtained figures with the declared characteristics of the model.
| Indicator | Normal | Alarm signal |
|---|---|---|
| Charging current | Corresponds to the charger power (for example, 1500-2000 mA) | Less than 500 mA with a powerful charger connected |
| Voltage | 4.2V - 4.4V during charging | Voltage surges or sharp drops |
| Capacity wear | Up to 10-15% for a used device | More than 25-30% capacity loss |
| Temperature | Not higher than 40-45°C under load | Heating above 50°C without active load |
Also perform a stress test: run a heavy game or benchmark (for example AnTuTu) with charging turned off. If the phone turns off when the charge level is 15-20%, the battery controller incorrectly reads the remaining energy, which indicates the need to replace the battery.
Checking wireless modules and sensors
A modern smartphone is a set of sensors and wireless interfaces. Failure of any of them can render the device useless. Start by checking Wi-Fi i Bluetooth. Try connecting to an available network and transferring the file to another device.
Pay special attention to the module GPS. Go outside or go to a window, launch maps (Google Maps or Yandex.Maps) and turn on geolocation. The phone should quickly (within 10-30 seconds) determine your location. If the search for satellites takes minutes or the point constantly “jumps”, the GPS antenna may be disabled or damaged.
Check the operation of the fingerprint scanner and face unlock system (Face ID). They should work instantly. To check the light and proximity sensors, you can use the engineering menu or application Sensor Test.
The lack of a GPS signal or constant Wi-Fi interruptions are often the result of poor-quality repairs, when the repairman forgot to connect the antenna cables after replacing the case.
Don't forget to insert your SIM card. Make sure that the phone can see the network, mobile data is working (4G/LTE/5G) and it is possible to make calls. Check both slots if the device is dual-SIM.
Cleanliness of the system and the presence of hidden locks
Before the final purchase, make sure that there are no other people's accounts on the phone. Go to settings and find the “Accounts” or “Users” section. If there is a Google account there that belongs to a stranger, ask the seller to log out.
This is critically important because of the protection system FRP (Factory Reset Protection). If you reset such a phone to factory settings, it will be locked and require you to enter the password for your previous Google account. Without it, it is very easy to turn the device into a “brick.”
Also check if custom firmware is installed on the phone if you are buying it as a new original. Log in Settings → About phone. The line “MIUI Version”, “One UI Shell” or build number must correspond to the official version for this model. The presence of strange firmware names may indicate interference in the system.
⚠️ Attention: If the seller refuses to reset the phone to factory settings in front of you or claims that “it’s better that the settings are saved,” this is a red flag. The real owner is not afraid of a reset.
Ideally, perform a full reset directly in front of the seller (Hard Reset). This will erase all data and show if the phone turns on cleanly, without prompting for passwords or locks. The path to reset is usually located in the menu Settings → System → Reset settings.
How to bypass FRP lock?
There is no official way to bypass FRP without a Google account password. There are paid service methods for professionals, but they do not guarantee results and may be illegal. It’s better to just refuse to buy such a phone.
Frequently asked questions (FAQ)
Is it possible to check a phone for viruses before buying?
Yes, but this does not give a 100% guarantee. It is best to perform a factory reset in your presence. This will remove any malware that the previous owner may have installed. After the reset, do not restore the data from the seller’s backup.
What to do if the seller prohibits checking the phone?
If the seller flatly refuses to check the functionality (camera, screen, calls), citing “session communication” or “many people willing”, refuse the purchase. With a 99% probability, the device has hidden defects that it is trying to hide.
How to distinguish an original iPhone from a fake Android one?
Fakes often give themselves away with their interface. Try going to the Google Play Store - it either doesn’t work on fakes or gives errors. Also check the operation of Siri (on fakes it’s just Google voice search) and the build quality (plays, fonts).
How much time should you devote to checking a used smartphone?
A high-quality check of all main components (screen, camera, communications, battery, sensors) requires at least 20-30 minutes. Don’t rush the process, thoroughness will save you money in the future.
Do you need to check the moisture protection when purchasing?
It is strictly forbidden to check the moisture protection by immersion - you can damage the phone. Look for moisture indicators (LDI) inside the SIM card slot. If they turn red, the phone has been in contact with water, and the warranty does not apply to it.