When a drive belt should be replaced in your car?

Your engine has a number of mechanical accessories attached to it, such as an alternator, water pump, power steering pump and an air conditioner compressor. A drive belt is the part that drives all these accessories. The belt itself is driven by a pulley attached to the engine crankshaft.
Drive Belt Drive belt
Most cars have one or two drive belts. When a car has only one drive belt, it might be called a serpentine belt. A drive belt is made of a durable rubber-based material. In most cars, a drive belt is replaced between 40,000 and 70,000 miles, however, we have seen cases where a drive belt lasted over 100k miles.

What can make a drive belt wear out sooner? Water, road dirt, excessive heat and oil leaks can cause the belt to deteriorate prematurely. For example, if the protective shield underneath the engine (engine undercover) is broken, the belt will need to be replaced sooner. Oil leaks can also damage the drive belt. In this photo, the belt is falling apart because it's soaked in oil leaking from the engine valve cover gasket. A worn-out drive belt tensioner can also cause the belt to break earlier.
Eventually, a worn drive belt will break. See the photo below. A broken belt can leave you stranded by the side of the road. This is because the engine will overheat if the belt-driven water pump is not running. Also, if your car has a hydraulic power steering, you will 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 electric systems. For this reason, some jurisdictions even include a drive belt into the list of items required to pass the vehicle safety inspection.

When should a drive belt be replaced? Most car manufacturers don't specify the exact mileage, but recommend having the belt inspected during regular maintenance visits.

How can you tell if the drive belt needs replacement? Your mechanic will look for cuts, cracks, missing chunks, glazing and other signs of wear; usually, it's fairly easy to see. For example, the belt in the next photo shows cracks and missing chunks of the grooves on the ribbed side.
Worn out drive belt This belt is worn out
See another example, the drive belt in this photo also shows cracks. It's also recommended to be replaced. The belt may also need to be replaced if it makes chirping or squeaking noises or if it's stretched. It is also a good idea to replace an old drive belt when an alternator or any other component driven by the drive belt is replaced, or when you prepare your car for a long road trip.

How long can a cracked drive belt last? There is no way of knowing that, it may last for a few months or it may break in a day or two.

To work properly, a drive belt must be under proper tension. Most newer cars have an automatic spring-loaded or hydraulic belt tensioner that doesn't need to be adjusted. 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 squealing or screeching noise that lasts for a few seconds after the engine is started.

Does the drive belt tensioner have to be replaced together with the belt? No, a drive belt tensioner only needs to be replaced if it's worn out or noisy or not working properly. Of course, if it's only a little extra and the tensioner shows some wear, it might be a good idea to replace it too as a preventative maintenance.
If a car has two drive belts, do both have to be replaced at the same time? It's not necessary, however often there is no extra labor involved in replacing the second belt. If you only have to pay extra $20-$40 for the second belt, why not get them both done at the same time?

Replacing the serpentine belt may cost from $75 to $250, depending on a car. Is it easy to replace the serpentine belt as a DIY project? If you have sufficient mechanical skills, proper tools and the belt routing diagram, it's not very difficult. Your parts store may help you with the routing diagram and tools.