Xiaomi Redmi Note 9 is one of the brand’s most popular budget phones, but even it isn’t immune to sudden crashes. A spontaneous reboot is a problem that many owners face, and it can be caused by both software bugs and hardware malfunctions. Unlike scheduled reboots (like after a system update), spontaneous reboots often signal serious problems, from the processor overheating to the failure of the board elements.
In this article, we’ll look at all the possible reasons why the Redmi Note 9 turns off and turns on on itself, from the banal overflow of cache to critical battery failures. You’ll learn how to diagnose a problem without visiting a service center, what settings to check first, and when to contact the wizard. We’ll focus on the hidden MIUI features that can cause crashes, and we’ll break down myths about “curing” the phone through factory reset.
If your Redmi Note 9 only reboots when you're doing something (like playing or charging), it narrows down the possible causes, and if it's chaotic, you'll have to test the system in a complex way, and let's start with the simplest, and then gradually get to the more complex.
1. Software failures: firmware or applications are to blame
In 70% of cases, the Redmi Note 9 spontaneous reboots are caused by software conflicts, which can be either a bug in the MIUI firmware itself or a failure in third-party applications.
System updates. Xiaomi regularly releases security patches and MIUI updates, but sometimes they contain critical bugs. For example, MIUI 12.5.3 and MIUI 13.0.4 for Redmi Note 9 have been reported to have massive complaints about random reboots, the problem was solved by rolling back on the previous version or waiting for a new patch.
Application conflict: Some programs (especially optimizers, antiviruses, or aggressive graphics games) can overload the processor or conflict with system services, such as Clean Master or DU Battery Saver, which are known to disrupt the MIUI background cache.
The system cache (/cache) and application cache (Dalvik-cache) become clogged over time, leading to freezes and reboots. On the Redmi Note 9, this is especially acute due to the limited storage capacity in the budget version (3/32 GB).
- 📱 Check the MIUI version: go to Settings → About Phone → Version MIUI. If the version is newer than 13.0.6 but the phone is released before 2021, compatibility issues may be possible.
- 🔄 Clear the cache: in Settings → Storage → Cleanup, select Cached Data and delete it.
- 🚫 Remove suspicious applications: sort programs by installation date and remove those that have started crashing.
2.Overheating: Why the phone turns off from high heat
The Redmi Note 9 is equipped with a MediaTek Helio G85 processor that is prone to overheating at high loads. If the chip temperature exceeds 90°C, the safety mechanism is triggered, the phone turns off or restarts to prevent damage to the board, which is one of the most common causes of spontaneous reboots.
How do you check the temperature?
Install the AIDA64 or CPU Monitor app and run the heating test. The normal temperature of the Helio G85 under load is 50–70°C. If the readings are above 80°C, this is critical.
- 🔥 Games and heavy apps: PUBG Mobile, Genshin Impact, or Call of Duty can heat up the processor to the limit.
- ☀️ External conditions: Using a phone in direct sunlight or in a stuffy room exacerbates the problem.
- 🔌 Charging + Load: If you play or shoot video while charging, the temperature rises by 20–30%.
Decisions:
Use Game Mode (Settings → Special Opportunities → Game Turbo Mode) to limit the background.
Remove the cover - it can block the heat sink.
Set FPS limits in games (for example, in PUBG, select Balanced Mode instead of HDR).
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If the phone overheats even in plain, check the fan in the charger (if you use wireless charging) - it can be clogged with dust.
3. Battery problems: wear or factory defect
The Redmi Note 9 battery, which has a capacity of 5020 mAh, can run for 300 to 500 charging cycles before it degrades, and if the phone turns off at 20 to 30 percent charge or reboots when it's connected to the grid, it's the battery that's to blame.
| Symptoms. | Probable cause | Decision |
|---|---|---|
| The phone turns off at 15-40% charge | Battery wear (capacity loss) | Battery replacement (cost) ~1500–2500 ₽) |
| Reboot when connecting charging | Short circuit in the power controller | Diagnostics in the service (possible replacement of food board) |
| The phone is warming and discharged quickly | Battery bloating or cell damage | Immediate replacement, risk of fire! |
| Failures at low temperature (below 0°C) | Lithium polymer batteries lose capacity in the cold | Use a heated cover or not operate in the cold |
How to check the status of the battery?
1. Enter the dialer code:
##4636##2 Go to the Battery Information menu and see Health Level. If it's below 80%, it's time to change the battery.
3. The alternative is the AccuBattery application (shows real capacity and wear).
What happens if you don’t replace the battery?
4 Conflicts with SIM- card or SD- card
Few people know, but a faulty SIM- card or microSD can cause Redmi Note 9 reboots. This is because the Communications Controller (Modem) and the memory controller work through the common MediaTek Helio G85 chip. If the card is damaged or formatted incorrectly, the system can "fall" with the mmc_read_fail error.
Signs of a problem:
- 📶 The phone is rebooted when you call or connect to the network.
- 💾 Failures occur when video is written to a memory card.
- 🔄 System logs (viewed via ADB) have E/MmcService(123] errors: read failed.
How do you check?
1. Remove the SIM- card and the SD- card, then use the 1-2 phone of the day. If the reboots stop, one of the cards is to blame.
2. Check the SD- card on another device (e.g., a PC via a card reader) if it is undetectable or gives an error, replace it.
3. For SIM- cards, try updating the operator profile (in mobile network settings).
☑️ Diagnostics of memory cards
5. Failures in the work of the motherboard
If all previous methods failed, the problem could be in hardware, such as chip damage, contact detachment, or battery failure. The Redmi Note 9 is particularly vulnerable in three circuit board zones:
1. Power Controller (PMIC) - responsible for the distribution of power. If it malfunctions, the phone may turn off when charging or not turn on at all.
Memory (eMMC) – if memory cells are damaged, the system falls with E/Storage(123) error: disk read error.
3. Antenna switch is responsible for the operation of SIM- cards and Wi-Fi. When it breaks, the phone is rebooted when you try to call or connect to the network.
How do you diagnose?
Connect your phone to your PC via ADB and check the logs:
adb logcat | grep -i "error\|fatal"If you have a string in your logs like E/PowerManagerService(123): reboot due to thermal emergency, overheating or power controller is to blame.
If the phone doesn’t turn on after rebooting, it could be a sign of the memory chip detaching (supplier required).
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Hardware board failures cannot be fixed software, and if the diagnosis shows PMIC or eMMC errors, contact the service with micro soldering equipment.
6.Influence of custom firmware and root rights
Many Redmi Note 9 owners install custom firmware (like LineageOS or Pixel Experience) or gain root access for advanced features, which often leads to unstable system operation, including spontaneous reboots.
Typical errors:
How do we bring stability back?
1. Remove all modules installed through Magisk and restart the phone.
2. If the firmware is custom, return to stock MIUI via Fastboot:
fastboot flash boot boot.img
fastboot flash system system.img3. Check firmware integrity through the MD5-sum (the downloaded file must match the official hash on the Xiaomi website).
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Before you run through Fastboot, turn off the Mi Account check in the developer settings (OEM unlock), otherwise the phone will lock!
7.External factors: charger, cable, moisture
Sometimes the reason for reboots is not in the phone, but in external devices or operating conditions.
1. Non-original charger.
Using cheap cables or power supplies with inappropriate parameters (5V/2A instead of 9V/2A) leads to voltage surges to which the phone reacts by rebooting. It is especially dangerous to use charging from other brands (for example, Samsung or Apple), since they have a different fast charging protocol (Quick Charge vs MI Fast Charge).
2. Moisture intrusion.
Even if the Redmi Note 9 is not positioned as waterproof, it has minimal splash protection (IP53).However, getting moisture into the USB-C connector or under the speaker can cause a short circuit, leading to a shutdown.
- 💧 The phone reboots when connected to a charger.
- 🔊 You hear wheezing or distorted sound from the speaker.
- 🔋 Charging indicator flashes red.
Mechanical damage.
A hit or fall can cause microcracks on the board that don't show up immediately, but after a few days or weeks. If the phone reboots when shaken or pressed on the body, this is a sure sign of mechanical damage.
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If the phone only reboots when connected to a specific charge, the problem is 90% of the time it is in the cable or power supply, not the device itself.
What do you do if nothing helps?
If you’ve tried all the ways, but the Redmi Note 9 still reboots itself, there are three options left:
1. Reset to factory settings.
This is a radical method that deletes all data, but often helps with software failures. Before reset, back up through Settings → Additional → Backup.
Settings → About phone → Reset settings → Delete all data.
2. Contact the service center.
If the problem is hardware (such as a board or battery malfunction), self-repair can make matters worse. Xiaomi's official service centers offer free diagnostics (in some cities).
- Battery replacement: 1500–2500 ₽.
- Repair board (ploughing PMIC) – 3000-6000 ₽.
- Replacement of the charging connector is 1000-1500 ₽.
3. Phone replacement under warranty.
If your Redmi Note 9 is warranty-free (usually 12-24 months) and the problem is factory-related (such as a defective battery or board defect), you are entitled to a free repair or replacement.
Save a check or confirmation of purchase (an electronic check from the online store is also suitable).
Do not open the case yourself - this will void the warranty.
Contact the authorized Xiaomi service center (list on the mi.com/ru/service website).
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Before visiting the service, reset your phone to factory settings and delete all accounts (Google, Mi Account) to speed up the diagnostic process.