My central air conditioner died at the start of triple digit temperatures. Should I repair or replace?

My central air conditioner is 20 years old, although I replaced a $600 part about 3 years ago. Is it time to replace the entire unit and if so, what brands are best in terms of reliability and "energy star" efficiency? Should I use single room units instead? I would be happy without having the whole house... show more My central air conditioner is 20 years old, although I replaced a $600 part about 3 years ago. Is it time to replace the entire unit and if so, what brands are best in terms of reliability and "energy star" efficiency? Should I use single room units instead? I would be happy without having the whole house cool, if that would be more energy efficient. I live in Pasadena, southern California, if that makes a difference.
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