Front Rotors on 2007 Jeep Compass

How could you tell when the rotors are worn?
Just to be rusty is a need of replacement?

Asked by for the 2007 Jeep Compass
The surface area around the perimeter will be rusty but as long as the area where the brake pads ride is smooth you should be ok
1 more answer
Have the run out of the rotor measured, depending on if the rotor is warped, it may be able to be machined, if the rotor is still thick enough to cut. The only real way to tell if a rotor is expired is by having it measured, and checked for signs of wear, like cracks, heat signatures, etc.