A PCV (Positive Crankcase Ventilation) valve is an important part of an engine's emission system. It helps remove harmful vapors from the engine's crankcase and redirects them back into the fuel system to be burned up in the combustion process. The PCV valve should be replaced according to the manufacturer's recommended maintenance schedule, which is typically every 20,000 to 50,000 miles, or if there are any signs of wear or malfunction, such as excessive oil consumption, rough idle or a check engine light that comes on.