Positive Crankcase Ventalation

The PCV valve is a variable orifice that controls the flow of crankcase fumes, admixed with fresh air admitted to the crankcase by the breather, into the intake tract. With no manifold vacuum, a restrictor—generally a cone or ball—is held by a light spring in a position exposing the full size of the valve's orifice to the intake manifold. -wiki

EGR Valve

Exhaust Gas Recirculation Valve

Exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline and diesel engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. - wiki

Oxygen Sensors

Oxygen Sensors, Air Fuel Ratio Sensors

 Measure the exhaust-gas concentration of oxygen for internal combustion engines in automobiles and other vehicles in order to calculate and, if required, dynamically adjust the air-fuel ratio so that catalytic converters can work optimally, and also determine whether the converter is performing properly or not.- wiki