Still having no communication to OBD 2 port 2005 PT Cruiser

I'm having the same issue everyone else is 2005
PT Cruiser touring non turbo...24.L  through 02
sensor low voltage... now I'm down a rabbit hole.
They say if the ignition switch doesn't generation
12 volts to PCM the PCM can't have an output
enough to give to the relays (fuel pump and ASD
relay) to in turn give the (ignition coil, spark plugs,
spark plug wires, crankshaft sensor and cam shaft
senor, oil sending unit, Fuel pump, fuel pump
sensor and 02 sensors upstream and down stream  
enough voltage to keep running and car dies ) Just
like mine is doing ! How do I know which one to
attack first and how do I measure voltage out of
each one and do I do it while the car is running or
key in on position only without Frying ny car
altogether? Help!!!