Positive crankcase ventilation (PCV) valves help the crankshaft ventilation system remove gases from the engine, and they can wear out and should be cleaned or replaced every 20,000 to 50,000 miles. A vehicle's owner's manual will direct you to the location of the PCV valve, and it will also contain recommended time periods between replacements. As your car approaches the 20,000 mile mark, you can pop the hood and check to see if the PCV valve is functioning properly.
For this assessment, the engine should be left running, but don't forget to put your car in park or set the emergency brake. There are two methods to quickly determine the health of the PCV valve. The first is to remove the oil cap and place a stiff piece of paper over the opening. If the paper is not sucked against the hole, this could be a sign that the valve is faulty.
Another method is to disconnect the valve from the valve cover, while leaving the hose attached. Place a finger over the open end of the hose and you should feel a strong suction, which indicates that the valve is working well. While it's detached, give the auto part a shake. An unobstructed valve will produce a rattling noise.