Q: I have a 2003 Subaru Forester and upon starting this morning it ran fine for a minute or two. Then the Check Engine Light came on.

Too much air flowing into the intake causes the engine to lean out too much air in relation to the fuel and misfire. At an idle, the PCV valve restricts air flow, to reduce this problem. At high manifold vacuum (idle), a spring loaded valve is drawn up and partially restricts flow to the crankcase. The PCV system is relatively simple. An inlet hose connects to a filtered air source. This is used to supply clean air that is drawn through the engine. Most of the time this air is supplied through the engine air filter.

On a few designs there is a separate inlet filter that cleans the incoming air for the PCV system only. This filtered air flows through the engine, picking up fumes and vapors. The air exits through another hose, connected to manifold vacuum. The flow of air draws fumes from the crankcase and burns them harmlessly in the engine. This also creates a slight vacuum, relieving any pressure that may build. Negative pressure helps to prevent oil leaks and oil consumption by the engine. The PCV valve also helps regulate the amount of air flow, which helps prevent oil being drawn out of the engine. When the PCV valve is not working correctly, this will cause an imbalance in air flow and vacuum causing the Check Engine Light to come on.

I would recommend having an expert from YourMechanic come to your location to diagnose your Check Engine Light firsthand and replace your PCV valve as necessary. Best of luck.