I made the mistake of resetting the the brake mileage when I changed my rear brakes and sensor. I ended up getting the "car on lift", brake warning light and "----" miles indicated for next brake change
After reading the different threads and Bentley manual stuff on this I finally found a way to fix this.
The system looks for two "untriggered" sensors when you reset the brake service mileage. If it sees even the preliminary trigger on any of the sensors it will give you this error. The preliminary trigger as stated above does not mean that there is no brake pad left. In my case the front pads had at least a little under 50% life left in them and therefore I did not replace them or the sensor. So this is what I did to fix it.
I took the old sensor I just removed from the rear and cut of the sensor portion. I then stripped the two wires back and twisted them together which creates a short and mimicks the reading a new sensor gives. Unplug the partially used sensor at the socket connection on the car (not the brake) and plug in the shorted old sensor. Do the brake reset procedure and you will get the appropriate mileage restored. Go back and remove the shorted sensor and plug back in the partially used sensor. At this point everything should be back to normal with the new sensor showing 90,000 miles and the partially used sensor showing whatever miles that were there before you created the error.
Hopefully this helps you. I think you should be able to use this procedure to reset the front or rear sensor one at a time without having to replace both sensors.