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I have a early 2015 Retina Macbook. I recently got a Dell 1707FP monitor to use as an external display and I have an Aukey CB-C26 hub that provides me with an HDMI port. I'm using an Amazon basics HDMI to VGI cable. When I connect the monitor, the laptop recognizes an external monitor is connected but the monitor is just a blank blue screen. I know the cable works because when I connect using the same cable to my Windows laptop the monitor works fine. I also know my Aukey adapter works when I connect it to my TV using an HDMI cable.

Here's the odd thing. If I restart my Macbook while it's connected to the external display, when the login page appears it appears on the external display. As soon as I put in my password, the display flickers and goes blue while the Macbook screen wakes up. So, the display is getting a usable signal at least some of the time. Any idea what is causing the blue screen on the external display and if there's anything I can do to change it?

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There are a number of possible causes and related solutions, but let's start with the most obvious solution first.

Reset your PRAM/NVRAM

Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). I suggest you reset this, here’s how:

  1. Shut down your machine. Yes, a full shut down, not just logging out.
  2. Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
  3. Hold those keys down until your Mac reboots again and you here the startup chime.
  4. Let go of the keys and let your Mac reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. mouse speed, time and date/timezone, etc).

Give this a go and report back with how you go.

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