IOS 9's cool new Wi-Fi Assist feature could drive up your cell phone bill

Hands on with the iPhone 6S
Hands on with the iPhone 6S

Apple's new iOS 9 operating system has an awesome feature called Wi-Fi Assist. You'll love it -- unless it hogs up your data plan.

Wi-Fi Assist is a new tool that lets your iPhone use 4G cellular data to help boost a weak Wi-Fi signal.

You know when you're outside your house or down the hall in your apartment building and your phone is still technically connected to your home network -- even though you can't get a signal? That inevitably means your streaming music cuts off, you can't use Google Maps to navigate when you get in your car, or that super important text just won't send.

Before iOS 9, your options were: 1) wait until your iPhone finally recognized that you were out of range from your router or 2) go to settings and shut off Wi-Fi (and then forget that you shut it off, which is super-annoying).

Now, when your iPhone recognizes that your Wi-Fi signal is kaput, it will automatically switch over to your cellular network to keep you connected. Neato!

The problem is, if you're doing something super data-intensive like watching Netflix or downloading apps with a weak Wi-Fi signal, Apple's new tool will start quickly eating away at your data plan.

Apple (AAPL, Tech30) has enabled the new feature by default, and it hasn't said much to tout Wi-Fi Assist. As a result, many people have complained about receiving surprisingly high cell phone bills after upgrading to iOS 9. There's no way to attribute higher monthly data usage directly to Wi-Fi Assist, but there's a chance it could be the cause of the sticker shock.

There's good news, though: Your iPhone will notify you when it has switched by graying out your Wi-Fi icon. So if you see a gray Wi-Fi notification at the top of your screen, stop downloading the entire contents of the Library of Congress.

Each of the four national carriers offers apps that will notify you when you're about to reach the limit of your data plan. Or you can get an unlimited data plan on Sprint or T-Mobile's networks.

You can also toggle off Wi-Fi Assist by navigating to Settings > Cellular > Wi-Fi Assist. But you'll be missing out on an extremely useful feature that will only rack up your data if you're not paying close enough attention.

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