Boot Loop Recovery
iPhone Boot Loop & Restart Data Recovery
A boot-looping iPhone shows the Apple logo, gets partway through startup, then restarts. The cycle repeats indefinitely. Your data is still on the NAND storage chip; the phone just cannot finish its boot sequence. Causes range from a corrupted iOS update to a missing sensor on an aftermarket flex cable to a hardware fault on the logic board. We diagnose which one, fix it without erasing your data, and copy everything to external media.
What Causes an iPhone to Boot Loop
Every iPhone boot follows a fixed sequence: the PMIC brings up power rails, the CPU loads the bootloader from NAND, iOS initializes hardware drivers, and the system checks that required sensors and components respond. A boot loop means the phone reaches some point in this sequence and then fails, triggering a restart. The NAND chip holding your photos, messages, and contacts is not damaged. It served data to the CPU during the boot attempt. The phone simply cannot get past whatever step is failing.
Three broad categories cause boot loops. First: software corruption. An interrupted iOS update, a phone that sat dead in a drawer for months and lost its file system state, or a failed jailbreak. Second: missing sensor detection. iOS expects specific hardware sensors to respond within minutes of boot, and forces a restart when they do not. Third: hardware communication failure. A damaged flex cable, a failed IC, or a broken solder joint prevents a component from initializing during startup.
A boot loop is different from a dead iPhone that will not turn on. A dead phone draws no boot current; the CPU never starts. A boot-looping phone has a working power delivery circuit. The CPU runs, the Apple logo appears, and the boot sequence progresses before something causes the restart. This distinction matters because the diagnostic path and repair approach are different.
The 3-Minute Restart Pattern
One of the most predictable boot loop patterns: the phone boots to the lock screen (or nearly to it), runs for about three minutes, then restarts. This cycle repeats every time. The timing is consistent because it is driven by a system process, not a random hardware failure.
iOS runs a daemon called thermalmonitord that monitors hardware sensors during operation. Certain sensors must report within the first few minutes of boot: the Prs0 barometric pressure sensor, Mic1 (bottom microphone), and on some models Mic2 (rear microphone). If any required sensor fails to respond, thermalmonitord crashes, and iOS forces a reboot to attempt recovery. Since the sensor is still missing on the next boot, the cycle repeats indefinitely.
You can confirm this pattern by checking the panic logs at Settings > Privacy > Analytics & Improvements > Analytics Data. Look for entries starting with “panic-full” that reference thermalmonitord. If the phone does not stay on long enough to reach Settings, the 3-minute timing alone is a strong indicator.
Which Sensors Cause the Restart, by Model
iPhone X / XS / XS Max
Prs0 (barometric pressure sensor) and Mic1 (bottom microphone), both on the charging port flex cable. A bad aftermarket charging port that omits the barometric sensor is the most common cause. The phone boots and runs until thermalmonitord queries Prs0, gets no response, and forces the restart.
iPhone 11 / 11 Pro / 11 Pro Max
Three required sensors: Mic1 on the charging port flex, Prs0 on the charging port flex, and Mic2 (REARMIC2) on the power button flex cable. Any one missing sensor triggers the restart. iPhone 11 models are the most common victims of the 3-minute loop because repair shops replace the charging port and do not realize the aftermarket part is missing Prs0 or Mic1.
iPhone SE (2020)
Mic1 on the charging port flex. Simpler sensor requirements than Face ID models, but the same failure mode when an aftermarket charging port omits the microphone component.
The root cause is almost always a bad aftermarket charging port flex that is missing one of these sensors, or a corroded flex cable where the sensor traces have broken. Replacing the flex with an OEM-quality part that includes all sensor components eliminates the loop.
Software Corruption vs. Hardware Failure
Many boot loops are pure software. An interrupted iOS update that left the file system in a half-written state. A phone that sat powered off in a drawer for months while the battery drained to zero, corrupting system files. A failed jailbreak that modified boot chain files. These do not require any hardware repair.
On a DC power supply (DCPS), a software-only boot loop has a distinctive current pattern: the phone ramps to 0.07-0.19A, holds for 20-30 seconds as the CPU attempts to load iOS, drops to zero when the boot fails, then ramps again as it retries. The current draw is normal for a boot sequence; the phone just never completes it. All power rails are healthy. No components are shorted. The CPU, NAND, and PMIC are functioning correctly. The operating system itself is the problem.
Step one for any software boot loop is an iTunes or Finder Update. Connect the phone to a computer, put it in recovery mode, and select Update (not Restore). Update reinstalls iOS while preserving all user data. Restore erases everything. This is the single most important distinction in iPhone data recovery.
Never select Restore on a data recovery phone. Restore erases the NAND and reinstalls iOS from scratch. All photos, messages, contacts, and app data are permanently deleted. If iTunes or Finder does not offer the Update option, use 3uTools with the “preserve user data” setting. Never let anyone (including an Apple Store employee) restore your phone if you need the data.
If Update fails or the phone will not enter recovery mode, the boot loop may involve hardware. But always attempt the software path first. A software fix takes minutes and preserves data. Jumping straight to board-level diagnosis wastes time when a simple Update would have resolved the problem.
One specific software-adjacent cause: speaker amplifier firmware incompatibility after an iOS update. Certain iOS versions change the speaker amp initialization sequence. If the speaker amp IC has outdated firmware, iOS hangs during audio subsystem initialization and restarts. Disconnecting the speaker flex and booting without it confirms this cause. The phone boots normally without the speaker connected, and you can extract data through the lock screen.
Battery Data Line Failure
iPhones communicate with the battery over a single-wire data line (SWI) that reports charge state, temperature, and cycle count. If this data line is broken, the phone boots but cannot read the battery state. iOS interprets the missing battery data as a critical fault and shuts the phone down after a few minutes. The timing varies but is usually under five minutes, similar enough to the 3-minute sensor pattern that it gets misdiagnosed.
iPhone 7: TIGRIS_BATTERY_SWI_CONN
The battery data line runs from the battery connector through the Tigris IC on a trace labeled TIGRIS_BATTERY_SWI_CONN. Physical damage to this trace (from a previous repair, a drop, or corrosion) breaks the data communication. The phone boots, shows the Apple logo, reaches the lock screen, then shuts down when iOS determines it cannot read the battery. Diode mode on the battery connector SWI pin shows the line is open. The fix is a jumper wire from the battery connector data pin to the Tigris IC, bypassing the broken trace.
iPhone 8: Q3200 MOSFET Failure
On iPhone 8, the battery data line passes through Q3200, a small MOSFET that gates the SWI signal. When Q3200 gets ripped off its pads (common during battery replacement if the technician pries too close to the component), the data line goes open. Diode mode on battery connector Pin 2 should read approximately 0.793V; an OL (open line) reading confirms Q3200 or the trace is damaged. The repair is a jumper wire bridging the input and output pads of Q3200, bypassing the damaged MOSFET entirely. The phone boots, reads the battery state over the alternate path, and stays on long enough to copy data.
Ear Speaker Flex and Face ID Boot Loops
On iPhone XS and newer, the ear speaker flex cable carries more than the speaker. It also carries the flood illuminator (part of the Face ID system) and the ambient light sensor (ALS). When this flex is damaged, connecting it to the board causes a boot loop. Disconnecting the flex and booting without it lets the phone start normally, at the cost of Face ID and the ear speaker.
The specific culprit is usually the flood illuminator or the ALS on the flex. A shorted or non-responsive component on the flex pulls down a power or data line during boot initialization. iOS detects the fault, fails to initialize the sensor subsystem, and restarts. This is the same flex that causes Error 4013 during iTunes restore. The failure mechanism is related: Error 4013 appears during a USB restore attempt, while the boot loop appears during a normal power-on sequence. Same damaged flex, different symptom depending on whether you are restoring over USB or booting standalone.
Diagnostic approach: measure resistance to ground on J5700 pins 33 and 31 (the flood illuminator data lines). Normal resistance indicates the flex is healthy. A short to ground or an open line indicates damage. Remove the flood illuminator from the flex and retest. If the boot loop stops, the flood illuminator is confirmed as the cause.
For data recovery purposes, the fix is simple: disconnect the ear speaker flex, boot the phone, enter the passcode using the on-screen keyboard, and copy the data. Face ID does not work without the flex, but the phone is fully functional otherwise. The phone does not need a permanent repair; it only needs to run long enough to transfer files.
Update vs. Restore: Preserving Your Data
This is the single most important concept for anyone dealing with a boot-looping iPhone. iTunes and Finder offer two options when a phone is in recovery mode: Update and Restore. They sound similar. They are not.
| Option | What It Does | Your Data |
|---|---|---|
| Update | Reinstalls iOS system files only | Preserved |
| Restore | Erases NAND, reinstalls iOS from scratch | Permanently deleted |
Many customers and even some repair shops do not know this distinction. They see a boot-looping phone, connect it to iTunes, and click Restore because it is the more prominent button. The phone stops looping because iOS is freshly installed. The data is gone.
If iTunes does not offer the Update button (this happens when the phone is in DFU mode instead of recovery mode, or when the iOS version is too old), use 3uTools. It has a “preserve user data” option that performs the equivalent of an Update even when iTunes will not cooperate. This is a standard tool in our shop for software boot loops where the phone enters DFU but not recovery mode.
If a repair shop offers to “fix” your boot loop by restoring it: Say no. A restore erases all user data. If they already restored it, the data is gone and cannot be recovered. Always confirm that any repair attempt uses Update, not Restore, before handing over a phone where data recovery is the priority.
Model-Specific Boot Loop Causes
Each iPhone generation has different hardware failure points that produce boot loops. Knowing the model narrows the diagnosis before we touch the board.
| Model | Common Boot Loop Causes | Notes |
|---|---|---|
| iPhone 7 | Tigris battery SWI data line damage; audio IC pad separation (“loop disease”) | Audio IC loop disease was a widespread defect: the audio IC pads separate from the board due to flex, causing boot loops and grayed-out voice memos |
| iPhone 8 / 8 Plus | Q3200 MOSFET ripped off pads; home button connector J5800 bent pins; Trinity EEPROM corruption | J5800 bent pins from careless home button reconnection cause intermittent boot failures. Trinity EEPROM stores calibration data; corruption causes a loop during sensor init |
| iPhone X / XS | Ear speaker flex (flood illuminator / ALS); charging port flex (Prs0 / Mic1) | Same flex that causes Error 4013. Disconnect to test; phone boots without Face ID |
| iPhone 11+ | Three required sensors (Prs0, Mic1, Mic2); volume connector corrosion; sandwich board separation; audio codec IC | Mic2 (REARMIC2) is on the power button flex, not the charging port. Volume connector corrosion from liquid exposure causes intermittent loops |
The iPhone 7 “loop disease” deserves a note because it was a design defect, not a user-caused failure. The audio IC (U3101) sits in a high-flex area of the board. Over time, the solder balls under the IC crack from repeated pocket flexion. The phone boots, attempts to initialize the audio subsystem, fails, and restarts. Symptoms also include grayed-out voice memos and no speakerphone. The repair is to reball or replace the audio IC after reinforcing the pads. For data recovery, a temporary reball is sufficient to boot and copy files.
Our Diagnostic Process
Boot loop diagnosis follows a specific order. Each step either resolves the problem or eliminates a category of failure, narrowing what remains. We do not skip steps because misdiagnosis wastes time and risks the board.
- Known good parts first: Connect a known good battery, screen, and charging port flex. This eliminates accessory failures immediately. A bad aftermarket charging port is the most common cause of 3-minute boot loops, and it takes 30 seconds to swap one in for testing.
- DCPS current draw analysis: Connect the logic board to the DC power supply at 3.8-4.0V. A software boot loop shows 0.07-0.19A ramp, 20-30 second hold, drop to zero, repeat. A hardware boot loop shows abnormal current patterns: too high (short circuit), too low (communication failure), or normal current but the phone restarts at a consistent point during boot.
- Attempt Update (not Restore): If current draw looks normal, put the phone in recovery mode and run an iTunes/Finder Update. This fixes all software boot loops without erasing user data. If Update resolves it, the phone boots, and we copy the data.
- Panic log analysis: If the phone reaches the lock screen before restarting, check the panic logs. A thermalmonitord panic confirms a missing sensor. The log names the specific sensor that failed to respond, pointing directly to the flex cable that needs replacement.
- Flex cable elimination: Disconnect flex cables one at a time and attempt boot after each removal. Ear speaker flex, charging port flex, power button flex, volume button flex. When removing a specific flex stops the boot loop, that cable (or a component on it) is the cause.
- Battery data line check: For iPhone 7 and 8 models, measure diode mode on the battery connector SWI pin. An open line reading (OL) confirms a broken battery data trace or a damaged Q3200 MOSFET. Jumper wire repair restores the data line.
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Frequently Asked Questions
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