I've heard several people say they had planned to re-use them when they bought new wheel bearing hub assemblies that did not include the sensor. Apparently, they are either hard to get loose or they are brittle because of the comments about them breaking when trying to remove them. However, it you are replacing the ABS sensor then maybe you don't care if you break the old ones. On that note, if you have a bad ABS sensor be prepared to pay within $40 or $50 bucks of what a complete wheel bearing hub assembly would cost including with a new ABS sensor. I was recently quoted around $128 for an ABS sensor by itself versus $170 for a brand new wheel bearing hub assembly that included a new sensor. Lastly, if you are trying to replace a "bad" ABS sensor because you think it will make the ABS idiot light turn off keep in mind that the sensor could actually be good but the wheel bearings are so worn out that the ABS sensor can not register properly.