Many times, you are given a bad alternator. Auto-zone is notorious for giving out bad alternators. When you bring your alternator in for a core trade in, they ship the bad one out for reconditioning. Often times, the reconditioning does not exactly recondition very well. If your alternator keeps going bad, talk to the comp0any or mechanic that is supplying you with them. There is no reason to except AND PAY for a bad alternator.