The sensor is usually just a bit of wire that makes an earth connection when it meets the brake pad, it's not uncommon for the wire to be damaged and if the metal back of the pad meets the disc it could mean a pair of new discs plus of course the inconveneince of an immediate replacement (as in stop and call the AA for a tow). If the sensors are working then even when the light comes on your OK to drive for a few days or even a week of so depending on mileage, it's often just cheaper and more convenient for the gagrage to do it during the service, unless you are prepared to get your hands dirty.