-you may be able to replace a faulty stem yourself
A faulty valve stem can cause your tires to release air and leave you with a flat tire. Having it repaired by professionals can cost upwards of thirty dollars, but in some instances, you may be able to replace a faulty stem yourself. If the wheel does not currently have a tire on it, the process if fairly straight forward and requires one inexpensive tool. However, swapping valve stems with a mounted tire in place requires some specialty tools that may cost more than having the repair done.
1.Insert the valve stem through its hole in the wheel.
2.Screw the valve stem tool onto the valve stem.
3.Use the handle to pull on the valve stem.
4.Use the handle to pull on the valve stem.
5.Unscrew the valve stem tool.