The positive crankcase ventilation system was one of the earliest emission-control devices. It draws leftover combustion gases from the crankcase (the oil pan and bottom of the engine) and routes them back into the engine, where they're burned in the combustion chambers instead of escaping into the atmosphere.
The PCV system is seldom listed as a maintenance item, but it can cause performance and emissions problems. A valve that's supposed to regulate the flow of these gases is the heart of most PCV systems (some newer vehicles don't have a valve). If the valve doesn't open and close on schedule, or if any part of the system clogs, the result can be a rough idle, sluggish acceleration or increased oil consumption.
The PCV valve is usually mounted in a grommet on a valve cover, at the end of a hose or tube. One way to check whether a PCV valve is functioning is to remove it and shake it. If you can hear a metallic rattling noise, it's likely in good working order.
Whether an engine has a PCV valve or not, a hose or tube in the PCV system may become clogged from built-up sludge, or a vacuum hose may leak, so it pays to inspect the entire system, clean it if needed and test the valve for air flow.
A clogged PCV system or inoperative valve can increase oil consumption because pressure builds when the vapors in the crankcase aren't allowed to flow into the combustion chambers. That additional pressure can force oil past seals and gaskets. If the valve is stuck in the open position, or there's a leak in the system, that will allow too much air into the engine and throw off the air-fuel mixture, likely triggering the check engine light.
Though it's not listed in the maintenance schedule in most vehicles, the PCV system should be inspected periodically to make sure it's still in good condition, especially if engine performance has deteriorated.