Idler pulleys are used to maximize belt contact with other pulleys and to redirect belt around other parts that might interfere with the belt.
there is one, but two if you count your belt tensoiner.
They very in price, depending on the car, from around $12 to $80.
only if the idler arm will not hold the tension on the belt and the pulley needs replaced when the are warn sharp or warn flat
The engine usually has an idler and/or tensioning pulley as well as a few pulleys that use the power of a traveling serpentine belt to drive components such as the alternator, a/c pump, etc. Any of the pulleys that have one or more grooves that assist to guide the belt(s) are, technically, sheaves. Idler or tensioner pulleys that are completely flat because the flat back of the belt rides on them are not sheaves, but are pulleys.
Take it to the dealer! It's not a job for the timid or any shop that doesn't know what they are doing. Make sure you get the idler pulleys and automatic tensioner replace along with the timing belt. If the bearings seize in those idler pulleys it will ruin the belt and in turn take out your engine.
In automotive terms, There are drive pulleys such as the drive pulley mounted to the crankshaft on most engines. There are driven pulleys such as an alternator, power steering, water pump, a/c compressor and all the accessories driven by the drive pulley. There are idler pulleys and tensioner pulleys. There are timing belt pulleys. And for non automotive related terms, There are block and tackle pulley systems. There are pulley hoists. Wire rope pulley blocks. Cable pulleys. Rope pulleys.
Worn or incorrect belt. Defective, glazed idler or pulleys.
take pressure off the belt by pulling back the spring tensioner. Remove old belt, put the new belt on all the pulleys but the idler pulley, pull the tensioner back slip belt on to idler, Look to make sure you have the belt on all pulleys good, your done.
Use a "straight edge" to make sure that the "valley" area (where the belt rides) is in alignment with the other pulleys
Disengage the idler arm lever ( it is a spring loaded arm located on your deck on left side). There is a bracket that holds it in place. Push arm forward over bracket then carefully allow arm to release all the way back. This will remove tension from belt. Then simply slide belt over pulleys keeping track of how they are aligned on pulleys. When installing new belt be sure belt is aligned in both front pulleys and be sure all belts are set in pulleys before reengaging idler arm lever.
The water pump and timing components should be replaced at intervals specified by manufacturer and you may replace just the idler pulley.