Diagnose and fix a leaky valve stem
You topped off the air in your tires, but a few days later, you notice that one of them is low again. You may have a hole in the tire, but before you take it in to the garage, first check for a leaky valve stem (Photo 1). If the stem is leaking, you can easily fix it yourself. You only need a valve core tool and a package of new valve cores (both inexpensive).
The fix is simple. Jack up the wheel. Then remove the old core with a valve core tool, screw in a new core (just snug it up; it's easy to strip the threads), and then refill the tire. But here are a few nuggets of advice. First, be sure to wear safety glasses when you pull out the old core. Debris or even the old core could hit you right in the peepers when the air releases. Second, look closely at the old core threads. If they show any sign of corrosion, clean the interior threads of the valve stem with the tap end of the valve core tool. And last, always use a valve stem cap to keep out the dirt! Dirt around the core is probably what caused the problem to begin with.