A "bad" idler pulley is one where the bearing is worn, causing the pulley to make noise and possibly bind or seize. If this happens, the belt can be thrown off and cause a loss of all accessories (like the power steering pump, alternator and water pump).
RepairPal Certified shops can diagnose and repair your 2007 Isuzu Ascender. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.