CV boots are vital for keeping your car in smooth running condition. They are connected to the drive shafts, which connect the transmission to the wheels. CV (short for constant velocity) boots are flexible rubber covers which protect the CV joint and keep the necessary grease from escaping and prevent dirt getting in and wearing out the joints prematurely. CV boots sometimes tear open, which allows the grease to escape and the dirt in. Damaged CV boots will cause and instant MOT or NCT failure. The good news is, replacements are extremely cheap, the bad news is that a lot of components have to be removed to change them so the labor charge to replace a cv boot is likely to be high.