Your Ford Dealer should be able to place their special tire tool against the tire to see if they get a reading. Then after replaceing the tires they will need to use the same tool to relearn the ecm to the new sensor's. If you don't have the sensor's than the little annoy light will come on. I don't know if that will effect any other funtions of the car. Some cut the timing in the engine just not sure if the Ford Focus does.
2013 Candy Blue Focus Titanium
2010 Flame Blue SVT F-150 Raptor
2007 Black Mustang GT/CS (SC'ed with 452rwhp)