How can I run diagnostics on my iPhone 6Plus that I am unable to start

You cannot. Only Apple has software to run diagnostics. If you cannot start the device, see this support document. If that does not help, then make an appointment at the Genius Bar of the nearest Apple store to have the device examined by Apple. If your iPhone, iPad, or iPod touch won't turn on - Apple Support

EDIT: Well, seems my response didn't go through right away. You can use this if you are too far from an Apple Store to see about mail-in service. Contact - Official Apple Support

Contact - Official Apple Support. My husband's iphone 6plus that is no longer in warranty the phone is completely froze up. Can do a hard reset and it will come back on but the screen is still froze up. Cannot do anything. AT&T said it is an apple problem. Let me know how they fix yours please.

Mine isn't broke. I was responding to help the OP. If you're having problems with your device, then make an appointment at the Genius Bar of the nearest Apple store and have the device checked. If it is out of warranty, they will check it for free, and then explain your options from there.

That is not possible.

I suggest you call your local Apple Store and set up an appointment for them to examine it. Afterwards they will tell you your options.

I wanted to diagnose my phone

FahmiKing wrote:

I wanted to diagnose my phone

Then make an appointment at the Genius Bar of the nearest Apple store.