Pelican Technical Article: 911 Carrera Engine Starting Troubleshooting
Basic non-start checklist posted above. I printed one out & carry it in the car. O2 sensor connects to your DME (see DME info in link above). I am not positive if O2 being disconnected would prevent starting, but does make for rough running. If yours has cat convertor, should have O2 sensor cable running to engine compartment.