When should a drive belt be replaced in your car?

Updated: September 06, 2021
The car engine holds several mechanical accessories, such as an alternator, water pump, power steering pump and an A/C compressor.
Drive BeltDrive (serpentine) belt. See also: Fig1, Fig2, Fig3.
The drive belt drives all these accessories from the engine crankshaft pulley (a.k.a. harmonic balancer); see the photo. Most cars have one or two drive belts. A single drive belt is often called a serpentine belt.

The drive belt is made of long-lasting rubber-based material. We have seen cases where a drive belt lasted over 100k miles. However, in most cars, a drive belt is replaced between 40,000 and 70,000 miles, as part of preventative maintenance. The reason is that an old, worn-out drive belt can break, often without any prior warning signs.
What happens to a car if a drive belt breaks? If a drive belt breaks, you won't be able to drive your car. This is because the engine will overheat if the water pump is not running. If your car has a hydraulic power steering, you will also lose the steering assist, as the power steering pump will be disabled.
Broken drive belt Broken drive belt.

In addition, the alternator will stop supplying electric power that is needed to recharge the battery and run the vehicle's electric systems. This, in turn, will cause the charging system warning light (red battery icon) Red battery icon to illuminate on the instrument panel. In fact, if the drive belt breaks while driving, the charging system warning light is the first possible sign that can alert the driver.

 Worn out versus new drive belt Old vs new drive belts.
When should the drive belt be replaced? Most car manufacturers don't specify the exact mileage, but recommend having the belt inspected during regular services and replace it if it looks worn out or damaged.

How can you tell if a drive belt needs to be replaced? Your mechanic will look for cuts, cracks, missing chunks and other signs of wear; usually, it's fairly easy to see if the belt is worn out. For example, the old belt in the photo above shows cracks.
Worn out drive beltDrive belt showing cracks and missing chunks. Fig 2.
This photo shows the belt with missing chunks of the tread material on the ribbed side. The belt may also need to be replaced if it makes chirping or squeaking noises or if it's stretched or damaged. One of the early signs of a worn out drive belt is when it starts squeaking in rainy weather.
Another common reason to replace a drive belt is when it's soaked in oil. Oil damages the rubber material, like in the next photo.
Drive belt damaged by leaking oil Drive belt damaged by leaking oil.
If a drive belt is saturated in oil, the oil leak will need to be repaired first. This is because if you install a new belt and the oil leaks onto it, the belt will not last long.

To work properly, the drive belt must be under proper tension. In older cars, the belt tension had to be adjusted manually in regular intervals. One of the symptoms of a loose drive belt is a loud screeching noise that lasts for a few seconds after starting the engine.
Many modern cars have an automatic spring-loaded or hydraulic belt tensioner that doesn't need to be adjusted.

Drive belt tensioner Automatic spring-loaded drive belt tensioner. Fig1, Fig2.
If the automatic spring-loaded belt tensioner is sticking or shows signs of excessive wear, it makes sense to replace it together with the drive belt; it takes a little extra labor. A hydraulic belt tensioner might need to be replaced if it's leaking oil.

Replacing the serpentine belt may cost from $75 to $250, depending on the car. Replacing the belt tensioner together with the belt will add $45-$155 to the total repair bill.

Does the drive belt have to be replaced when an A/C compressor or alternator is replaced? No, it's not necessary. However, if it doesn't take extra labor and the old belt shows some wear, it might be recommended.

Is it easy to replace a serpentine belt as a DIY project? If you have sufficient mechanical skills, proper tools, a belt routing diagram and follow the service manual instructions, it's not very difficult.
In some cars, a special tool might be required to release the belt tensioner. Your parts store may help you with the routing diagram and special tool rental. Youtube offers plenty of videos on the belt replacement.

If the car has two drive belts, do both belts have to be replaced at the same time? No, it's not necessary. However, in most cases there is little extra labor involved in replacing the second belt. If you only have to pay $20-$40 extra for the second belt, why not get them both done at the same time?

What causes the drive belt to wear sooner? Water, road dirt, excessive heat and oil leaks can cause the belt to deteriorate prematurely. For example, if the splash shield under the engine (engine undercover) is broken or missing, the belt will wear out sooner. A failed drive belt tensioner will also cause the belt to wear out sooner or break/slip off.