Mid 2011 21.5" Mac Desktop Will not start up.

[chuckben]

When powering up I get 1" wide green vertical stripes 1" apart across the entire screen (See attached photo). The grey background looks normal and has the Apple logo and a working progress bar. After a few minutes it appears as if it is going to start but the screen goes solid grey, then goes black. Then the same power up sequence repeats, and will continue until it press the power button to power down. When it was running it was running very slow, but everything worked.

With your suggestions I am prepared to try to fix it. It appears the schematics are not easy to come by. Or, if I can be sure the LCD display is working I can convert it to a monitor with a LCD interface kit ($25 on AliExpress).

 

[Nguyen Duc Hieu]

When powering up I get 1" wide green vertical stripes 1" apart across the entire screen (See attached photo). The grey background looks normal and has the Apple logo and a working progress bar. After a few minutes it appears as if it is going to start but the screen goes solid grey, then goes black. Then the same power up sequence repeats, and will continue until it press the power button to power down. When it was running it was running very slow, but everything worked.

With your suggestions I am prepared to try to fix it. It appears the schematics are not easy to come by. Or, if I can be sure the LCD display is working I can convert it to a monitor with a LCD interface kit ($25 on AliExpress).

The picture showed the typical issue with this model: failed GPU.
The GPU is a detachable MXM card.
You can replace the card with a working one, or try rework it with a hot air gun, or bake it in your oven. Search Youtube "Rebake iMac GPU" for relevant videos.

 

[theMarble]

The picture showed the typical issue with this model: failed GPU.

Yep. All 2011 Macs with dedicated Radeon chips (in this case, the 6750M or 6770M). A re-bake won't do much in the longer time since it will fail again.
It can be replaced with a MXM-A card, cheap Metal-supported options are the GTX 765M and Quadro K1100M.

There is a wiki on MR that tells you all about it.

 

[TheShortTimer]

A re-bake won't do much in the longer time since it will fail again.

It's also a method that comes with significant health and safety risks.