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I want to fix my Mac by myself My screen is black so I can’t do the hardware giagnosis Is there any guide to hardware diagnosis?
See what the diagnostic LEDs are telling you, here’s a guide on doing that: How to find and read the Diagnostic LEDs
If your computer starts/bongs but there is no image at all it is likely to be the GPU (Graphic Processing Unit). This is a common fault on this model. If this is the case only an expert would be able to put it right. i.e. half the cost of a new one to put right. I take it you have reset the NVRAM/PRAM? https://support.apple.com/en-gb/HT204063
