Q: How to remove and replace the PCV valve and hose in a Toyota Prius?
A: The PCV valve is threaded into the left (driver's) end of the valve cover, with the PCV system valve installed in the end of the valve cover. For 2004 and later models, it is necessary to remove several components to gain access to the PCV valve. Start by disconnecting the negative cable from the auxiliary (12-volt) battery. Then, remove the two mounting bolts from the interfering engine compartment relay block and unclip the wiring harness retainers. The air filter housing should be removed next. Disconnect the two interfering injector and coil wiring harnesses and set them aside. Remove the plastic cover from the wiring harness between the valve cover and the inverter/converter coolant reservoir. Disconnect the PCV valve hose and remove the PCV valve, then reattach the hose to the PCV valve and reconnect the wiring harnesses. With the engine idling at normal operating temperature, place your finger over the end of the valve. If there's no vacuum at the valve, check for a plugged hose or valve and replace any plugged or deteriorated hoses. Turn off the engine, remove the PCV valve from the hose, connect a clean piece of hose and blow through the valve from the valve cover (cylinder head) end. If air will not pass through the valve in this direction, replace it with a new one. When purchasing a replacement PCV valve, make sure it's for your particular vehicle and engine size and compare the old valve with the new one to ensure they're the same.