Well Andy, it is possible to have a defective rotor which can be an imperfect casting. Heat-cool cycles stress the built in metal distortion and continue to make it warp. One of those metallurgical mysteries. If you replace both and the calipers are applying balanced braking force, you should be OK.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.