If you drop your iPhone in a puddle or spill a glass of water near it, the first thing you probably do is check to make sure it's working. You may power it off for a couple of days or place it in a bag of uncooked rice to absorb moisture. Whatever your course of action, when you turn it on, you are relieved to see that it works. In reality, a water-exposed iPhone may seem to work fine at first, but that can change. In the days that follow, your phone may behave as if it has headphones plugged in, even when it doesn't, which is caused by water in the headphone jack or charging port. You may be able to repair an iPhone stuck in headset mode using cotton swabs and compressed air.
iPhone Stuck in Headphone Mode
If you know your phone was exposed to water, you are already aware that water damage is probably the culprit. To confirm your suspicion, check to see if your phone has been compromised by water. Apple places a white indicator in the SIM port of its phones to detect water damage. If that indicator is red, your phone has come into contact with water, and it's possible some of that water is in the headphone jack or charging port.
Fixing Water in Headphone Jack or Charging Port
If your iPhone is a model 6S or earlier, it's possible you may have water in the headphone jack. Newer iPhones beginning with iPhone 7 don't have a headphone jack, but they have a charging port, also called a lightning port, that can be damaged. Water in the charging port can cause issues with the phone's speakers, including causing them to degrade.
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If water is in the charging port or headphone jack, turn off the phone and use a cotton swab to soak up any water inside the port. You'll need to remove most of the cotton on the swab to fit it into the port.
iPhone Stuck Due to Software Issues
If your phone is still stuck in headphone mode after you tried removing water from the headphone jack or charging port, you may have a software issue. It's possible your phone is connecting to a Bluetooth device without you realizing it. You can find this information in your Settings app. Look for any connected Bluetooth devices and disconnect them to see if that fixes things or turn off Bluetooth completely.
Try turning off your phone and powering it back on, if you haven't already done that. A phone that seems to be stuck in headphone mode could be the result of a case that covers the jack. Remove the case and see if the issue persists. You can also try playing a song and then plugging in your headphones while it's playing. When you unplug the headphones again, the song may come through your speakers as it did before your phone was exposed to water. If you still can't fix the problem, an Apple authorized reseller may be able to take a quick look and use an expert tool to get things working again.