Q: Have MBP 15R..can't start system with external monitors plugged in

Hello GayleG,

Thanks for using Apple Support Communities.

If your displays are completely blank when powering on your Mac, then I suggest that you go through the troubleshooting in the article below.

Check connections

When using an external display be sure to check the following:

1. If you're using an Apple notebook, confirm the AC power cable or adapter is securely connected to the computer and the cable providing power to the display is also secure. It is always good to have your notebook connected to AC power when an external display is in use.
2. Confirm display adapters are fully seated in their respective connections and that they are supported models and for the computer and display. Refer to these articles to assist you with adapter compatibility and further configuration information: 
    
   • Monitor and Display Adapter Table
    
   • About Mini DisplayPort to HDMI adapters
    
   • Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)
    
   • Thunderbolt ports and displays: Frequently asked questions (FAQ)
3. Remove all display cable extenders, KVM switches, or other like devices and retest to determine if the issue is resolved.
4. Try unplugging the video adapter or cable and then plug it back in.
5. If more than one video adapter is in use (or "daisy-chained"), troubleshoot by using only one adapter. 
    
   • Example: A mini DisplayPort to DVI adapter connected to a DVI to HDMI adapter is an unsupported configuration because there is a series of adapters in use.
6. If available, try using a different display and or adapter (or use a different connector by using DVI instead of VGA, for instance).

Reset the system

You can reset the Mac's parameter RAM and SMC.

Reset the resolution

Start by resetting the Mac's parameter RAM. If the display does not come up, was previously set to an unsupported resolution, and still results in no video:

1. Start up in Safe Mode.
2. From the Apple () menu, choose System Preferences.
3. Choose Displays from the View menu to open the preferences pane.
4. Select any resolution and refresh rate that your display supports.
5. Restart your computer.

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support

Take care.

Hi Gayle,

Since the issue didn't show itself in safe mode, then there's likely an issue with a login item on your Mac.  

If an issue doesn't happen in safe mode

If an issue doesn't happen when your Mac is started in safe mode, try restarting again without pressing any keys at startup. If the issue appears to be resolved when you start up normally, it was possibly caused by a cache or a directory issue with your startup disk that safe mode fixed.

If you restart your Mac normally and an issue comes back when you reach your desktop, try disabling any login items that automatically open when you log in.

Try safe mode if your Mac doesn't finish starting up - Apple Support

Test your login items

1. Choose Apple menu > System Preferences, then click Users & Groups.

2. Click your account name below Current User, then click Login Items.

    

   Make a list of the login items—you’ll need to remember them later.

3. Select all of the login items, then click the Remove button .

4. Choose Apple menu > Restart.

5. If this solves the problem, open Users & Groups preferences again, add the login items one at a time, and restart your Mac after adding each one.

    

   When the problem occurs again, follow the steps above to remove only the last login item you added.

OS X El Capitan: If you think you have incompatible login items

Cheers.

Hi Gayle,

For further support with this issue I suggest that you get in contact with another one of our support resources by using the link below.

Apple - Support - Product Selection

Take care.