Signs of bad rotors include vibration when the brake petal is depressed, and a visual inspection that reveals wear and tear. Rotors can be replaced or resurfaced unless they are under a certain thickness. A rotor that is too thin does not absorb the immense heat of the braking process and may crack or break while in use, causing brake failure.
According to experts at Tomorrow's Tech, "The bottom line here is this: if a customers vehicle has a pedal pulsation, the rotors need to be resurfaced or replaced. You also need to measure rotor runout on the vehicle to determine if the problem is the rotor or the hub, so the appropriate corrections can be made."
Runout occurs when the rotor wobbles as it spins and can be measured with a dial indicator.
Rotors should be replaced if hard spots or cracks are present but can be resurfaced when they are grooved. Hard spots and cracks indicate inevitable rotor failure and resurfacing is not recommended for these conditions. Grooving occurs naturally during the braking process over a period of time and, assuming that the rotor is of an appropriate thickness, most motorists choose to resurface rotors rather than replace them initially.