Tire Pressure Senors problems

I have a slow leak from my valve stem. About 4 months ago I had to have the two front Pressure sensors replaced and now the shop where I get my tires said the valve core is leaking . Does anyone know if it's as simple as removing the old valve core/stem and screwing in a new one or is their some MAGIC involved where the dealer has to do it. Loses about 2 pounds a week before we got hit by Polar Vortex..

If thats the case all you need to do is unscrew the valve core and put a new one in.

the odds are when you unscrew the valve that the sensor will fall into the tire. EVERY one of these i have replaced has had to have the tire dismounted. of course everyone i have ever replaced was also damaged by taking the tire off.