both...flat tire warning and system wouldnt register pressures....if one goes does that mean it affects all tire sensors?

 

On an '09' Cayman S I had something similar. I'd get a low tire warning, then a flat warning all followed by dashes and no numbers. Mine did this two or three times and it never happened again.

 

If the batteries have run down and the sensors need replacing, then yes, all 4 would go within some window of time since all have seen similar in service time.

The TPMS sensors are, like batteries, brake pads, wipers, belts, and tires, to be replaced at after certain amounts of time. Probably best to put on a new set when you get 4 new tires mounted every 5+ year interval (at however many tire sets that is for you)