The document discusses emission control systems including exhaust gas recirculation (EGR) systems, positive crankcase ventilation (PCV) systems, and catalytic converters. It describes the purpose and function of these systems to reduce emissions and explains methods for diagnosing faults. EGR systems recirculate exhaust gases to lower combustion temperatures and reduce nitrogen oxide formation, while PCV systems recirculate crankcase vapors to be burned in the engine cylinders. The document provides details on EGR valve operation and diagnostic testing procedures.