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Many Android tweaking and hacking guides warn that you'll void your warranty by continuing. But will you actually be denied repair service if you've rooted or unlocked your bootloader?
This is a tough question to answer. There's what manufacturers say in warranty agreements, what's actually enforceable in court, and what manufacturers actually do when it's time to get warranty service. We're no legal experts, but we'll answer this question from our own experiences and what we've heard.
NOTE: Keep in mind that we're talking about rooting your phone or unlocking its bootloader--not unlocking it from your carrier. Most carriers will unlock your phone to use it on another network for you, which never voids your warranty. Unlocking your bootloader is a different beast.
What Does the Manufacturer Say?
Manufacturers are often eager to say any sort of unapproved software modification will void your warranty in the fine print. The rules are often different for Nexus devices or "Developer Edition" devices, even if manufacturers don't really really spell it out. Here's an unusual example of a Motorola representative clarifying the issue a bit in a public forum:
"The new (2015) Moto X Pure is not a developer edition, so unlocking the bootloader does void the warranty...
To sum up and clarify:
Unlocking the bootloader will show your warranty as void.
However, if an unrelated physical material failure should occur, such as a bad volume rocker or a failed speaker, it will be covered if the phone shows no signs of physical abuse. The key is that the problem can't be traced to software or abuse...
The above guidelines are applicable in the US only. Policies differ by region/country."
So for most phones, yes: even though many manufacturers offer an official way to unlock your bootloader, they still claim this kind of customization could void your warranty. They're generally more lenient with developer edition devices, though, which are designed to be hacked around with.
The language on Google's Nexus devices has also changed over time. Older Nexus devices used the phrasing "Yes, Unlock bootloader (and void your warranty)" while newer devices use the phrasing "Yes, Unlock bootloader (may void warranty)." One Reddit user asked a Google support representative, only to find that rooting and installing a custom ROM on his Nexus 6P would not void his warranty. But that's just one support representative, and this isn't really spelled out anywhere officially.
What Actually Happens When You Need Warranty Service
With manufacturers being so vague about their policy, it's hard to know what'll actually happen if you need service. While there's no hard and fast rule, most manufacturers will fix hardware-related issues without any fuss (much like Motorola stated in their policy above).
For example, if you're having problems with the screen, or your hardware buttons aren't working properly, it's likely that the manufacturer will just go ahead and fix the problem. This is especially true if your problem is a well-known issue with the device in question--like loose the loose headphone jacks on the original Motorola Droid. That's clearly a hardware problem that couldn't have been caused by rooting or installing a custom ROM.
In other cases, it may just not be worth the trouble for them to find out what if you've rooted it. If your device dies and won't boot, it's unlikely the manufacturer will attempt to perform forensics on the device to see if your bootloader was unlocked. They'll likely repair the device or replace it under warranty. As always, a little courtesy goes a long way too.
On the other hand, But this makes perfect sense. Manufacturers and cellular carriers--if you purchased the phone from a carrier--don't want to deal with customers who've rooted their phones or installed a custom ROM and have gotten themselves into trouble. The representative at your local AT&T probably won't answer questions about why such-and-such hardware feature doesn't work under CyanogenMod, and they shouldn't have to.
If You Caused the Problem, You're Out of Luck
However, there's a difference between a clear hardware defect and a problem you caused. If they try to boot the device and they see a logo for a custom ROM before it fails to boot, there's a good chance they'll tell you you're on your own. (Of course, if you can boot the device up and see that logo, there's a good chance you can fix the device yourself with a bit of research).
Remember, rooting and flashing ROMs comes with all sorts of dangers if done improperly. Maybe you flash a custom ROM and erased your wireless radio, or you did something wrong and it won't boot properly. If you take the device to the manufacturer or carrier and expect them to fix it, they'll throw up their hands and say it's not under warranty and you're on your own. Of course, that's a bit like installing Linux on a PC that came with Windows--you can't expect the manufacturer to support software you installed on it yourself.
In very rare cases, this kind of tinkering can "brick" your phone, rendering it completely unbootable. If that happens, and you tell the manufacturer that you were trying install a custom ROM, they won't want to fix it for you. Though again, we should note: truly bricking your phone is pretty rare, and in most cases the phone will at least turn on, meaning you can salvage it with the right research.
It's also possible to do things that could damage your hardware with that root access. Maybe you overclocked your phone's processor a little too hard and overheated it, for example. Such damage won't be covered under warranty, just like accidental damage caused by dunking your phone underwater or dropping it face-first on the pavement wouldn't be.
If You Need Service, Unroot Your Device First
Provided you haven't caused any serious hardware issues, like those in the above section, you may still be able to get warranty service even if you've technically voided it by rooting. We've had good luck with getting warranty coverage on our devices, even though they were rooted, unlocked, or had previously run a custom ROM.
If your device is still mostly working, it's a good idea to unroot it before sending it to your manufacturer for repairs. If you're using a custom ROM, you should restore the original ROM the device came with and re-lock the bootloader.
Some devices have a sort of "flash counter" that becomes triggered if you ever unlock the bootloader and flash a custom ROM, and a manufacturer may check this. They'll most likely do this if the phone has some sort of hardware problem that looks like it was caused through such modifications.
But if the phone has a hardware problem that's clearly the manufacturer's fault--and especially if it's not currently rooted, unlocked, or running a custom ROM--they'll often fix the problem. At least, that's what's happened in our experience.
So, what's the answer? It's a bit of a gray area. In general, as long as your device doesn't have hardware problems that look like they were caused by you messing with it and isn't running some weird custom ROM when you send it to the manufacturer or your cellular carrier, you're probably fine. There are never any guarantees, but it's always worth a shot.
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