Q: how do you know if you need to replace rotors

When brake pads are replaced, the brake rotors MUST either be re-surfaced on a brake lathe OR the brake rotors must be discarded and new rotors installed. If the old rotors are NOT worn below the legal minimum thickness, and are not otherwise damaged (scored or warped), the least cost option is to re-surface the rotors on a brake lathe. Most aftermarket rotors are junk made in China. If you buy new rotors because the old ones cannot be re-surfaced, I strongly recommend you install ONLY the OEM rotors from the dealer or rotors from a reputable name brand such as AC Delco. The reason that rotors can never be re-used as is, no matter how they appear to the eye and no matter what your present braking experience is, is because a variation in thickness in the rotor that exceeds just .001 inches WILL be felt as pedal pulsation and vibration. In comparison, a human hair is FOUR TIMES that value (a hair is .004 inches thick). In other words, the brake rotor must be perfect and I can guarantee that if the rotor is used, it is far from adequate for re-use (without re-surfacing at minimum) with brand new brake pads. In addition, once the rotor is re-installed on the wheel hub, the total installed rotor run out must NOT exceed .002 inches. Ask your mechanic to use a dial indicator to confirm that and if run out exceeds .002 inches the rotor must be re-indexed, shimmed or replaced (or the cause of the run-out found and repaired). Certified mechanics from YourMechanic install new pads and rotors on a mobile basis thus saving you a lot of money. If you desire that service please request brake service - new pads and rotors and the responding certified mechanic will get this taken care of for you. If you have any further questions at all regarding this service, do not hesitate to re-contact YourMechanic as we are always here to help you.