You can learn how to repair electronics by going to collage for that. or you can use google.com and do research on how to fix them. youtube.com can also help for video tutorials
First you find out what the car is doing, then you find out why it's doing what it's doing. You find out what part on the car is defective and then you replace it.
Depends on whether a person is mechanically inclined or not. If not it is cheaper by far to pay for repairs. Then, if mechanicaly adept, follow instructions for the repair. Buy the Haynes or/&...
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