In a very calm voice, I would apologize for the bad experience. I would explain that I would do everything within my power to resolve the issue. I would request further information from the customer to get exact information on product use and installation along with make and model of vehicle. I would check that the product sold matched the vehicle. If product was a starter, battery, or alternator I would test the product to insure proper working condition. If deemed a faulty product I would offer a new/ different product, if customer found that unacceptable and refused all prior offers I would offer a refund with profound apologies.