Used phones sometimes have faulty hardware that might escape basic inspection. That’s why I recommend running top-to-bottom diagnostics on the phone before you hand over the cash. Thankfully, Android phones have secret codes for doing just that.
A few years back, I bought a second-hand Android phone whose fingerprint scanner stopped working after a while. It worked as expected during the initial purchase, but when I brought it home, it wouldn’t pick up my fingerprint. Turns out, the scanner only stays active a short while after rebooting the phone. In retrospect, I should have learned whether used phones are worth buying in the first place.
I had also seen friends and family pick up phones with dead pixels, dead touch zones, and all kinds of networking issues. Since then, I’ve been using secret Android codes to test used phones. While secret codes aren’t the only way to test phones—you can find plenty of apps on the Play Store to do the job—I recommend you use them for a few reasons:
The testing tools contain hidden settings that aren’t available in the regular settings app. To launch them, you need to punch a secret code into the dialler app. Since these tools aren’t user-facing, the codes are not officially made public.
Some of the codes can be slightly different depending on the manufacturer of your phone. We'll be using a Redmi phone in this guide, which shares the same code with Xiamo and POCO phones. Here are a few of the codes for popular Android brands to open the "Diagnostics Mode."
You can find the code for your device by searching for “secret Android codes + the device brand name.” Look for codes that trigger diagnostics mode, engineering mode, or CIT tools. Once you have the code, you can run tests on each hardware component. All that might sound overwhelming, but this guide will walk you through every single step.
To start, type the diagnostic code into the phone’s dialler app, and it will automatically take you to the testing tools. This will likely look different depending on your phone model, but similar options should be available.
The very first thing you want to check is the hardware tags. Look for the IMEI number. Every phone has a unique identifying number called the IMEI number, which makes it easy to blacklist lost or stolen phones. Phones with blacklisted IMEI numbers become unusable.
Tap “Check Version Info” to reveal the phone’s IMEI number (dual-SIM phones have two IMEI numbers). Note the number and enter it into an IMEI testing tool such as Swappa or Imeipro.info. If the IMEI is clean, tap the “Pass” button and move on to the next check.
On the main menu, tap “Keyboard Test” to test the physical buttons on the phone. When you press a button, it should light up green. Faulty buttons will appear red. The phone I’m testing has three buttons, all working perfectly.
Once satisfied, tap “Pass” to automatically move to the vibration motor test. Tap “No Vibration," “Vibration Interval," and “Continuous Vibration” to make sure the motor works in different modes. Tap “Pass” to mark it.
The diagnostics app should automatically take you to the Backlight Test after the last step. Here, you can test the minimum and maximum brightness of the display. Turn the flashlight on and off too.
After that, you’ll land on the touch sensor test, which is just a bunch of blocks that fill in when you touch them. Next up is the color test. Touch the screen to start the test and scroll through the solid colors. You’re looking for any bright or dark spots on the screen.
Once you’ve made sure that the colors are showing up consistently, tap “Pass.” And with that, you’ve successfully tested the display.
The best way to test the receiver and the mic is just to make an actual call. But if you don’t have a SIM card handy, simulating a call is the next best thing. Simply tap “Receiver,” hold the phone up to your ear, and tap the numbers read aloud to you. Tap “Pass” to load the speakerphone test.
Once again, you’ll press the numbers read out loud to you. Mark it “Pass,” and you’ll get the mic test. Say something close to the mic and see if the phone picks up the sound.
Newer premium phones don’t have those anymore, but 3.5mm headphone ports are still common among budget Android phones. Tap “Headset Test” and connect a wired headset. It’ll automatically record ambient sound using the headset mic and play it back through the headset. You can also test the headset buttons here. Take the headset out to complete the test.
Now go back to the main menu and tap “Charger Test.” Plug in a charger and take note of the reading. A functional charging port should output close to or more than 1 Ampere.
If the current reading is below 1A, the cable itself might be faulty, so I recommend testing with a different cable just to make sure.
Testing the fingerprint scanner is pretty straightforward. Just tap the “Fingerprint Sensor Check” and check if it comes back green. This phone has a faulty fingerprint reader, which returns a red result. You can also tap “Fingerprint input test,” enroll your fingerprint, and test if it’s read correctly.
Next up is the proximity sensor. Tap “Proximity Sensor,” cover the front of the phone with your hand and watch the value change. If it doesn’t change, the sensor is broken and it won’t turn off your screen when you pick up a call.
To test the accelerometer, just tap “Accelerometer” and note if the readings are changing.
The light sensor automatically controls your screen brightness. To test it, tap “Light Sensor,” bring the phone close to a source of light, and wait for the DAC value to change.
To test the Wi-Fi and Bluetooth, just tap Wi-Fi AP Scan, and it’ll automatically check the Wi-Fi and Bluetooth hardware. Ensure that you see four “Pass” tags on the Wi-Fi and Bluetooth menus.
You’ll need a separate code to check the battery's health. On Xiaomi, Redmi, and Poco devices, open the dialler app and type *#*#6485#*#*. Once again, you can search “battery health secret code + the phone’s brand name” to find the right code for the device you’re testing.
Look for the reading in front of the MB_06 tag, which indicates battery health. It’ll read “Good," “Excellent," or “Poor." Poor battery health means that the phone’s battery is at the end of its life cycle and needs replacing.
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