My wifi drops when I plug in an external monitor through the thunderbolt port

Only thing that worked for me was using VGA instead of DVI,

strange thing is i had no problems for a month after i first plugged in my external DVI monitor.

MBP late 2011

OS 10.9.5

Im sad to have to say that my Early 2015 MBP Retina 13" (running El Capitan) shows exactly the same problems when connection via HDMI to a ASUS VX24A - no wifi unless I'm in 720p - it supports up to 2560x1440 - main intention was to use it for graphics on the mac when at home!!!

Tried changing my Sky router from auto to both channels 1 and 11, no such luck there either 😟

I had this problem, but managed to fix it by switching to an HDMI cable between laptop and monitor!

Thank you so much. After 3 hours I found that connecting to external monitor was causing the issue, when using the displayport. Finally connected to the internet now.

Actually that did not fix the issue.

Finally figured out a way to make this work. Good news all you poor souls in torment 🙂. I covered my cable with some aluminium foil and voila wifi started to work. I had to adjust the foil so as to cover tightly the head of the mini DP, and form a sort of cone around the cable(away from the head).

First, changing the channel numbers from auto to 1 through 7 did not fix the issue for me. Changing the resolution also did not help. Taking the laptop max distance away from the monitor also did not help.

Configuration :

Macbook Pro 13 El Capitan

LG 27UD68

Verixon MI424WR router

StarTech.com 6 ft Mini DisplayPort to DisplayPort 1.2 Adapter Cable M/M

The root cause of this is indeed interference generated by the DP cable, in my opinion.

The reason why swithcing to channel 1 works for many is that using channel 1 causes the 5Ghz spectrum to come into play. However, this will help only if you have a router which support the dual bands of 2.5 and 5 Ghz. Mine supports only 2.5 Ghz. Fios quatum has dual band routers according to google.

Not sure why even apple DP cable gives this issue for some.

There you have the solution which worked for me. I wonder if I should try out a apple cable as well or just get a better router. Having aluminium foil sticking out next to MBP is not what I would like.

Well I tried all of your solutions guys but none worked for me...

My ISP don't let me change the channel of WiFi

* MacBook Air (Mid 2012)

* OS X El Capitan 10.11.5

* 3rd party HDMI adapter

* LG TV

The cable and TV works fine on non-Apple computers (both windows and Linux). Shame I have access to only one mac.

My complain is that everything was running fine last week, but since Sunday I've got the speed drop issue.

Any Idea?

Already tried to change resolution, unplug-plug, reset router (config blocked by ISP), deactivate Thunderbolt networking :-(

This worked for me!! Thank you so much. I was going insane.

I fixed it by changing the wireless channel too. Option Click the wireless icon in task bar, then Window menu > scan and choose the Best 2.4Ghz recommended channel. In my case channel 2. Then changed the channel in my router's internet page which was 192.168.1.1 for me. Thanks for the thread. Still an issue in 2016. What a joke.

I have this same issue. Just bought a brand new 13" MacAir i7, Dell 28" S2817Q Monitor, and a Henge Dock. Excited evaporated with this unforeseen issue. +20mps drops to less than 4mps when plugging in monitor. Closed lid, in dock, out of dock, open lid, makes no difference. Is it a Thunderbolt thing? This is not a coincidence. C'mon Apple how 'bout a solution instead of a finger point!

omg the foil works! Thank you so much.

I fixed it simply by changing the position of hardware on my desktop.

From mbp > lg4k > wifi router

To lg4k > mbp > wifi route

All hardware is on the same line, near the wall: so I think that display was stopping wifi signal before arrive to mbp.

I hope my magic helps someone.

Bye

This is such a bizarre problem, but this absolutely fixed it for me. Plugging in the cord immediately killed my wireless until I changed the wireless channel to 7.

Changed my xfinity router channel to 1 and it worked for me