What do you mean by "seizing up"? The fans will normally come on once the car has run a while and warmed up. If they do not come on at all there can be a problem with the fan motor, an electrical connection or maybe just a fuse. Typically if the fans do not come on at all, you'll experience overheating, especially while driving slow or at a stop - since there will be little air movement. You're at risk for overheating your engine - which can result in permanent damage.