The purpose of the idler pulley is to ensure the belt is routed around the engine components smoothly and without obstructions. A faulty idler pulley can cause damage to the belt and cause components to not work properly. A bad pulley can be replaced in no time with the correct tools.
When the engine is idling, a bad pulley may make a squealing sound. This is due to the bearings in the pulley going bad. The bearings may also make various other sounds such as clattering or even a rumbling sound, making the vehicle sound as if there was much more wrong than a bad pulley. The sound can be centralised to the faulty pulley to make troubleshooting easier.
The bearings in a pulley may cause the pulley to freeze or in some cases hard to spin. The best way to determine if the pulley is not moving freely is to take the belt off the pulley and spin it by hand. If the pulley is hard to turn, then it will require replacement. Use caution taking the belt off the pulley. Ensure the vehicle is turned off and the key is out of the ignition.
Watching the belt as it travels over the pulleys will also determine if a pulley has gone bad. The hardware in the pulley may have deteriorated or the hole in the middle of the pulley may have got bigger allowing the pulley to wobble or not move smoothly. The pulley may work loose or keep pushing the belt off the pulley. This will indicate the need for a new pulley.
In some cases the pulley is fine and does not need to be replaced but the bracket holding the pulley in place may have got warped or loose. This will cause the pulley to wobble or force the belt off the pulley path. The pulley is held on by a mounting bracket that is secured to the engine. When the bracket is loose, replacement can be done in no time.