iPhone turns off by itself: shutdown or panic?
Updated 2026-10-14
If an iPhone turns off by itself, first check whether it left a panic-full log. No log usually means a power cut: battery, battery connector or charging circuit. A log means a kernel panic: a battery sensor (TG0B, TG0V), a thermal watchdog or another signature you can read and follow.
Is it a shutdown or a panic?
"It turns off by itself" covers two different situations, and the first job is to tell them apart.
- A real shutdown: the screen goes black and the phone stays off until you press the power button or plug it in. The power was cut, or iOS shut down to protect itself.
- A panic followed by a restart: the screen goes black, then the Apple logo appears on its own. To the customer it looks like "it turned off", but iOS actually hit a kernel panic and rebooted.
The quickest way to tell: open Settings › Privacy & Security › Analytics & Improvements › Analytics Data and look for panic-full files whose time matches the incidents (How to Get the Panic Full Log From an iPhone).
| Observation | Probable situation |
|---|---|
| No panic-full file at the time of the incident | Power cut or protective shutdown |
| Panic-full file at the time of the incident | Kernel panic: read the signature |
| Phone stays off until you press the button | Shutdown (battery, power) |
| Apple logo comes back on its own | Panic and automatic restart |
Why does an iPhone shut down without a log?
When the power disappears suddenly, the kernel has no chance to write anything. The most common reasons:
- Worn battery: under load (camera, games, cold weather), the voltage drops and the phone cuts out, sometimes with plenty of charge still displayed.
- Battery connector: a bad contact, bent pins or corrosion can cut the power for a moment.
- Charging circuit on the board: if the battery and its connector are good, the fault may be on the logic board side.
Check the battery health, the connector, and test with a known-good battery before going further.
Which panics look like a shutdown?
Battery sensors (TG0B, TG0V, TB0V)
The logic board reads battery data (temperature, voltage, fuel gauge) through dedicated lines. When that data stops arriving, the log shows a watchdog panic:
userspace watchdog timeout: no successful checkins from thermalmonitord
… Missing sensor(s): TG0B
thermalmonitord waited for battery readings that never came, and iOS restarted. On recent models, the same problem appears as an SMC sensor array code (for example 0x41 on iPhone 14 Pro, 0xa1 on iPhone 15 / 15 Pro, 0xa9 on iPhone 16 / 16 Pro).
A useful clue: if the battery percentage is missing or stuck at 0–1 %, the board has lost the battery data. On the iPhone 11 Pro / Pro Max, the battery connector goes through the charging port flex, so test that flex too.
The thermal watchdog
thermalmonitord is the iOS service that monitors temperatures. When it times out without naming a missing sensor, the iFixit wiki considers it generally a software problem: update or restore iOS before touching any part.
Overheating
A critical SoC temperature alarm can appear in the log. It is tied to a line between power management and the CPU: either the CPU is heating because of another component on the board, or the line is faulty. That is a board problem, not a replaceable part. A thermal camera helps find the component that heats, starting with the Wi-Fi and audio areas if the board was reworked.
What should I check, in order?
- Look for panic-full logs at the time of the incidents.
- If there is a log, read its signature (How to Read an iPhone Panic Log (Panic Full Guide)) or drop it into the tool.
- If there is no log, focus on the power: battery health, battery connector, a test with a known-good battery.
- If the phone is hot without reason, check the board with a thermal camera.
- If it restarts in a regular cycle, see iPhone Restarts Every 3 Minutes: Why and How to Fix.
What should I tell the customer?
Be clear about which case it is. "Your iPhone loses power" and "your iPhone crashes and restarts" lead to different repairs and different prices. Show them the log, or the absence of a log, and explain that the first test is a known-good battery or part, so they pay for the right repair.
What about non-genuine batteries?
A replacement battery shows a "non-genuine battery" warning unless it is an Apple part set up with System Configuration or Repair Assistant (iOS 18+). The warning by itself does not turn the phone off, but a poor-quality cell can. Fluctuating percentages or a random "non-genuine" message can also come with battery data panics.
For any restart pattern that is not a clear shutdown, continue with iPhone Keeps Restarting? Find the Cause in the Log.