A common brake question: if one rotor is bad, do you have to replace both on that axle, or can you just do the one? The strong recommendation is to replace rotors in axle pairs. Here is why.
Why replace rotors in pairs
Braking works best when the left and right sides of an axle behave identically. If you put a brand-new rotor on one side and leave a worn one on the other, the two can differ in thickness, surface condition, and how they grip and shed heat. That mismatch can cause uneven braking - a subtle pull, uneven pad wear, or inconsistent feel. Replacing both keeps the axle balanced and predictable.
Pads and rotors together
For the same reason, fit fresh pads when you replace rotors, and always as a full axle set. New pads bed in best against fresh rotor surfaces, and matched components brake evenly. See when to replace rotors and the bedding guide.
What about front vs rear?
You do not have to replace all four rotors at once - front and rear wear at different rates, and the fronts usually need it first. Do the axle that needs it, both sides, and leave the other axle until it is due. See front vs rear brake pads.
Are there exceptions?
Rarely - for example, fresh damage to a single new rotor from a specific cause. But as a rule for normal wear, always pair them. The small extra cost of the second rotor buys even, safe, predictable braking. Fit fresh hardware too, and if you are unsure, have a professional inspect the brakes.