You can replace them or you can reuse them. I too did my brakes just before they wore into the sensors. I reused the sensors. It took me 5 minutes and I pocketed the $8. They can be somewhat brittle, but if you carefully take them out and reinsert them into the new pads, there is no problem reusing them. The choice is yours.

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95 E320 Cabriolet, 169K