1998-2005 Carreras, the engine air-oil separator is located at the top left rear corner of the engine.
Air-oil separator are a emissions device that collects residual gases and vapors contained inside the crankcase and funneling them back into the intake manifold. This reduces the overall emissions of the Porsche engines.
We recently had a customer bring in his 2002 Porsche Carrera 4S because he was seeing drops of coolant on his garage floor. What happened was his Air Oil Separator cracked near where the coolant lines run into it. What can also happen is it can crack and cause oil to suck into the intake.. this can damage the sparks plugs and other sensors. AOS Porsche problems should be resolved ASAP.