A outer CV gaiter is a strong rubber protective cover which fits snugly over the CV joint in order to protect it from debris and damage when the vehicle is in motion. It contains lubricant which enables the parts of the CV joint to move smoothly without wear.
Failure in a outer CV gaiter will most likely be caused by damage to the rubber material which can mean that the essential lubricant is lost and contaminants can enter the confines of the gaiter which causes risk of damage to the inner CV joint. The CV joint is essential in driving the vehicle forward as it transfers torque from the transmission to the wheels at a constant speed, and if damaged can cause the vehicle to cease operation.
The most common way to diagnose a damaged outer CV gaiter is by inspection by a mechanic who will be able to see any signs of cracking, wear or lubricant leaking from the gaiter. However, in the event of a damaged outer CV gaiter causing damage to the CV joint you will hear a knocking noise coming from the vehicle, which becomes louder on acceleration.