On cars with PATS, there is a chip in the key that communicates with the ignition. If the key has no chip or the wrong chip, no ignition.
Mine was faulty once and my key would not disable the PATS. They had to replace some sort of module to fix it.
The keys also have to be programmed to work with your car. I think you have to have at least 2 working keys to do that though (maybe it is one). Should be outlined in owner's manual.
