Stock stems and caps are on all tires and I have checked the valves for leaks. I am originally from NY and the car was purchased there. But its toward the end of summer and when winter hits the car was parked and the TMPS was always on. I also thought it was a bit strange that all 4 tires are losing pressure. I don't think that I could possibly get 4 nails in 4 separate tires, what are the odds? I guess I will have to let the dealer figure it out.