Remove the PCV valve from the valve cover. Most valves are pressed into the valve cover but some are threaded and screw into the cover. Remove the hose from the valve. Take note of which end of the valve was in the manifold. This one-way valve must be reinstalled correctly or it will not function. While the valve is removed, the hoses should be checked for splits, kinks and blockages. Check the vacuum port (that the hoses connect to) for any clogging.
Remember that the correct function of the PCV system is based on a sealed engine. An air leak at the oil filler cap and/or around the oil pan can defeat the design of the system.