Explain the operation of the EVAP system. The EVAP system is in charge of recovering the excess fuel vapors and re-introducing them into the engine. It isn't just an emission system. If it malfunctions is also has a huge effect on engine performance and certain components operation will be cancelled by the ECM if the EVAP system fails. Learn how they operate here and how to test. The OBD-II scanner or Scan-1 is used to diagnose these systems and evaluate the process. See how it works.