Injectors get clogged over time, especially if you get one bad tank of gas, or if you let Ethanol type gas sit for over 3 months. It goes bad... Witchhunter.com or Lindertech.com will refurbish fuel injectors to like new. The injectors do not usually fail, the injectors internal microfilter gets clogged, or the injectors internal valve gets clogged, and a regular in the tank injector cleaning will not do squat to clean these.