Performing a visual inspection of a car part is usually the first step in assessing its condition. In the case of the serpentine belt, any sign of physical damage to the belt means the belt should be replaced, and may indicate the need to more closely inspect the driven components, tensioner, and belt pulleys.
But relying on a visual inspection of your customer’s serpentine belt alone is a mistake.
Since the late ’90s, serpentine belts have been made with ethylene propylene diene terpolymer, or EPDM. This synthetic material does not show visual wear until it is long past its useful life. Instead, you need to think of belt inspections more like a tire inspection.
Therefore, when inspecting a belt, gauge the wear of the ribs and grooves of the belt much like you check the tread depth of a tire. Do this with a special tool like the one shown below.