There is a problem with a sensor or modulator. You have a potentially dangerous situation because the ABS on one wheel may not work while the ABS on the others will. If that happens, it can cause your car to pull/veer hard and unexpectedly toward the side of the car that the faulty ABS is on. Not to mention, it can increase your stopping distance and reduce your ability to control the car. Take it in to a qualified mechanic. Since ABS has been required by law for several years now, almost any reputable shop should be able to correctly diagnose and correct the problem.
