A bad positive crankcase ventilation valve causes various symptoms, including ignition of the check engine light. Other symptoms include odd noises from the engine or oil on the air filter element.
An ignited check engine light is an indicator that the driver needs to have further diagnostic tests done on the engine. Checking the engine with a scan tool often returns a P0171 or P0174 code when the PCV valve is bad. These codes indicate the engine is operating with a lean fuel to air ratio, due to the blocked valve. Other potential codes indicate a bad fuel injector or mass-air-flow meter.
The failed valve causes some cars to produce a low moaning noise and others to make a high-pitched whine or whistle. Temporarily blocking the PVC vacuum source and listening for changes in the sound or the elimination of it helps to narrow the cause of the noise.
With some vehicles, a bad PVC valve results in random oil leaks. In cars where the valve uses the carburetor filter as its air source, the bad valve causes oil stains on the filter element.
Sometimes the problem is not with the valve itself, but with the grommet or hose. A split grommet or collapsed hose often mimics problems with the valve itself.