I need new brakes, and yes I have one warped rotor on the right front wheel. I understand replacing both front rotors but the dealership says all 4 rotors need replacement even though NONE of them have grooves. 2018 Platinum Ford Explorer is this true?
Brakes work by creating friction: hydraulic calipers squeeze pads into rotors, which removes material from both the pads and the rotors. Too much heat can cause rotors to warp, which causes a vibration when braking. Rotors should be replaced when they get too thin, which they measure using a specific tool, or are too warped to correct, which also is measured using a tool. Sometimes warped rotors can be resurfaced if they have enough remaining material and it's not too severe.
Brake rotors should generally be replaced in pairs, both fronts or both rears, in order to have even braking. If you had one thin and one thick rotor you may get more braking action from one side than the other, which isn't good.
This is a relatively new car, unless you are driving a heck of a lot I wouldn't expect your rotors to need replacing yet, so the dealer telling you all 4 need replacing is a bit dubious. The front 2 I can believe, the back 2 not so much.
Grooves are NOT the determining factor for rotor replacement. The two main concerns are runout (i.e. is the thing warped or not) and thickness (i.e. is the rotor worn to the manufacturer's service limit, or will it be too thin after being turned to remove any problem areas).
You cannot tell this by looking but you can measure the thickness with calipers or a micrometer and check the runout with a runout gauge.
Both figures, runout and thickness, are specified in the vehicle's service manual.
If you have these tools, do the measurements yourself. If not, then you're at the shop's mercy. If you don't trust the dealership, I can't imagine why you took it there to begin with.
In my experience, however, vehicle manufacturers today leave very little "meat" on the rotors to begin with. I'm sure this is to cut costs to some degree but more so to cut weight as every gram matters when figuring fuel economy numbers. Making the rotors a few 10th of an inch thinner may will a bit of weight. So I've noticed that the rotors almost always need to be replaced when the pads are worn. Having a warped rotor is a sign of hard use, so I'm not at all surprised that your rotors need to be replaced.