Once any stage of the sensor is tripped, you must replace it to get an accurate reading again.

You reset the service reminder via the instrument cluster, and with a new wear sensor it will go back up to the default/calculated value (in my case I think it was 34 or 36k miles for the rears).

If you haven't tripped a stage, then I don't see why a service reset on the brakes wouldn't bring it back to full mileage estimate. That said, you'd be tossing the pads with about 40% of their remaining life if the first stage hasn't tripped.

I used aftermarket sensors - seemed to work fine. If the sensors have continuity in them, they won't work (the car will see it as a tripped sensor). I got front and rear aftermarket sensors for less than half the cost of one OEM sensor, so I figured why not give them a shot. One side worked, so I'm already ahead there.