The PCV valve is internal within the valve cover and therefore, the engine cover has to be discarded so as to access the PCV valve. The fixed orifice for 2004 and later models (except 2018-later 4.3L models) is built into the valve cover and is not replaceable, while the 2018 and later 4.3L models use a conventional PCV valve that is screwed into the top front of the left side of the valve cover. For 2003 and earlier models, with the engine idling at normal operating temperature, the valve cover should be pulled from the rubber grommet with the hose attached and will enable that vacuum check by pulling out the PCV valve. For 4.3L engines where the PCV was produced after 2018, the PCV valve comes with a hose attached, and can be unscrewed from the left valve cover. To check if there is vacuum, block the valve opening with the finger and if their is no vacuum, check for blocked hose, manifold port or the valve, and replace any spoilt hose. When the engine is switched off gently move the PCV valve to listen for a rattle; if the valve doesn't rattle then it should be replaced. For the models up to 2003 and for 4.3L models manufactured before 2018, the valve must be pulled out from the end of the hose with the attention to the position of the valve and for the models which have been produced from 2018, the PCV quick-connect fitting must be disconnected to isolate the valve from the hose. A replacement should be of the correct vehicle and engine size as the old one; the two valves should also be compared. The new valve should be driven in up to the rubber grommet and then the grommet tested for damage and replaced if the case were applicable. Last but not least, for 2018 and later 4.3L's, the PCV was supposed to be threaded into the left valve cover and tightened securely by hand, further, the PCVs were supposed to be inspected for clogs or crimped hoses before connecting the quick-connect fitting onto the top of the valve until the until an audible click is heard.