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Your iPhone shows the Apple logo, goes dark, shows the logo again, and never finishes starting up. It's called a boot loop, and it's one of the more alarming things a phone can do — but before you panic or wipe it, two things you need to know: your data is almost certainly safe, and a factory reset usually won't fix it — and may erase everything for nothing.
We recently had an iPhone 13 come in stuck in exactly this loop — Apple logo, restart, repeat, endlessly. The owner had tried every reset they could find online. The culprit turned out to be something most people (and plenty of repair shops) never check: a corroded proximity sensor from a bit of liquid that had crept in through the earpiece. One tiny, water-damaged sensor was dragging the entire phone down.
And it wasn't a one-off. We see this same fault across most iPhone models — because nearly every iPhone puts the proximity sensor on a flex cable at the top of the phone, sharing a data line with other components. When liquid corrodes that sensor, it can take down the whole phone the same way, whether it's an iPhone X, an 11, a 12, a 13, a 14, or newer. This is the story of why that happens — and what to do if it's happening to yours.
An iPhone starts up in a fixed sequence: the power management chip brings up the internal power rails, the processor runs its built-in startup code, each stage loads and verifies the next, and then iOS runs a hardware self-check — confirming the components are all responding — before it finishes booting.
A boot loop means the phone gets partway through that sequence, fails the check, and restarts — then does it again, and again. The important part: it's failing during startup, which means the storage chip holding your photos, messages, and data is never touched. That's why your data is safe even though the phone won't start.
It's worth telling three similar-looking problems apart, because they're different faults:
Search "iPhone boot loop" and most advice tells you to put the phone in Recovery or DFU mode and restore it. Please be careful here. A boot loop is very often a hardware problem — and no software reset can fix a hardware fault. Worse, a Restore erases everything on the phone. So you can wipe all your data and still be stuck in the loop, because the physical cause is untouched.
There's one safe software step worth trying first: a force restart (press and release Volume Up, then Volume Down, then hold the Side button until the logo appears). That can clear a one-off software glitch and doesn't erase anything. If it doesn't hold — if the loop comes right back — the cause is hardware, and the drastic data-wiping steps won't help. Stop there and get it diagnosed. (An iTunes/Finder Update — not a Restore — can reinstall iOS without erasing data if the issue is genuinely software, but on a hardware boot loop even that won't stick.)
Here's the part that catches almost everyone out. Your iPhone's proximity sensor — the little sensor near the earpiece that turns the screen off when you hold the phone to your face on a call — sits on a flex cable at the top of the phone, alongside the earpiece and the front sensor array.
That sensor talks to the rest of the phone over a shared data line called the I2C bus — a single bus that several components communicate on. And that shared design is exactly why a tiny sensor can take down the whole phone: if the proximity sensor shorts out, it shorts the shared bus — and a short on that bus fails the hardware check that the boot sequence depends on. The phone can't finish starting up, so it restarts. Every time. That's your boot loop.
In our iPhone 13, the sensor had shorted because of corrosion — and corrosion means one thing: liquid got in.
And because almost every iPhone is built this way — proximity sensor on a shared flex at the top of the phone — this same fault shows up across the lineup. We've traced boot loops to a corroded proximity sensor on iPhone X, 11, 12, 13, 14, and later. The model changes; the mechanism doesn't.
You don't have to drop your iPhone in a pool for this to happen. The path of least resistance for liquid is the earpiece speaker grille — that little mesh slot at the top of the screen. It's an opening by design, and moisture finds it easily: a phone used in the rain, a steamy bathroom, a sweaty pocket, a screen wiped down with a damp cloth or hand sanitizer. The liquid wicks through the mesh and settles on the metal contacts of the earpiece and proximity sensor.
Then the slow damage begins. Water leaves corrosion on those contacts, and corrosion keeps spreading over hours, days, and weeks—long after the phone feels dry. The phone might work fine for a while, then start boot looping later, seemingly out of nowhere. That "out of nowhere" is corrosion that finally bridged the sensor's contacts and shorted the bus.
And yes — this happens even though modern iPhones are rated water-resistant. Water-resistant is not waterproof; that rating fades as the phone ages and its seals wear, and the earpiece grille is a known weak point. (More on liquid damage in our full iPhone water damage repair guide.)
The proximity sensor is the culprit people never expect — but in our shop, the battery is just as common a cause of a boot loop, and for a reason that makes sense once you see it.
Starting up is the most power-hungry moment in a phone's day: the processor spikes its current draw as iOS loads. A healthy battery delivers that surge without flinching. A failing, aged, or swollen battery can't — its voltage sags under the load, the phone browns out mid-boot, and it restarts. Then it tries again, sags again, and you've got a loop. A dead-empty battery can do the same thing on some models.
We see a few battery-related versions of this:
The takeaway: a boot loop is a power and hardware story. A corroded sensor shorting the bus and a battery that can't hold voltage produce the same symptom on the screen — which is exactly why guessing (or wiping the phone) doesn't work, and why the diagnosis below matters.
The reason this fault fools people is that the phone looks physically perfect — no cracks, no obvious water. The diagnosis is a process of isolation:
That isolation step is the difference between a real diagnosis and guesswork — and it's why "just factory reset it" so often wastes people's time and data.
Once we've confirmed the proximity sensor, there are two paths. There's also a real trade-off worth knowing about on iPhone X and later (X, 11, 12, 13, 14, and up) — the models that use Face ID. On older Touch ID iPhones, the boot loop happens the same way, but this particular Face ID trade-off doesn't apply:
This is exactly why it matters where a water-damaged phone gets repaired. A shop that just swaps parts may end your boot loop but kill Face ID. We do the board-level cleaning under a microscope in-house, which saves the original sensor — and Face ID — whenever the corrosion hasn't gone too far. When it has, we lay out your options plainly (accept the Face ID loss, or pay for the OEM-programmed cable) and let you choose. Either way, you'll know exactly what your phone needs and what it means for Face ID before we start any work.
And through all of it: your data stays put. Because this is a startup/hardware repair, not a software wipe, your photos, messages, and everything else come back with the phone.
If your iPhone is stuck on the Apple logo and restarting, bring it to any of our three Chicago-area stores. We diagnose boot loops at the board and component level — isolating whether it's the proximity sensor, the battery, storage, or a deeper board fault — and we work to fix it without erasing your data. The diagnostic is free, and we'll give you an honest read on what it needs and what it means for features like Face ID before we start any work.
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