What does a red light in the headphone jack mean?

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The red light is optical audio output.

There is a tiny switch inside the headphone/optical audio port that can get stuck.

Read the advice from user noondaywitch in this thread:

https://discussions.apple.com/thread/3684038

The red light is the digital in light, that is how it reads the headphone jack. The reason the sound doesn't work is because a switch to turn the speakers back on is stuck (note that if you boot the computer, you will hear the startup tone).

There are several ways to fix this...

• Some have found that putting the headphone plug in and out several times (maybe twisting it a little) will work.
• Others have used a toothpick to adjust the switch (I don't advise this because the toothpick may break).
• Instead of a toothpick, many have used a paperclip (probably best to use an insulated one). I couldn't get this to work, but I would suggest straightening, folding in half, then twisting the paperclip and moving it around in the port.
• You can also use a sewing pen/needle.

I have seen all of these methods online, but they didn't work for me. I took a que from this instrucable and cleaned my jack with a dry q-tip. I then cut off most of the end and turned it around inside the port. This worked, but after I put the headphones back in the same problem came up, I just did the same procedure again and it worked again (even after I plugged the headphones back in and took them out).

PS: I woud suggest putting some q-tips and possibly some scissors in your MacBook bag in case the problem arises on the move.

this website helped me out. I had to try it a few times though.

https://josephhall.org/nqb2/index.php/stuckonmute