Everything on your vehicle that was equipped by the manufacturer serves a purpose, big or small. Something as small as a tire valve stem cap may seem insignificant, but it actually serves a pretty big purpose. The valve stem cap is not just a little plastic or metal cap to make the valve stem look nice. It is designed to protect the Schrader valve, a valve stem into which a valve stem core is threaded, that keeps the air or nitrogen in your tires.
A valve stem cap is important for the Schrader valve because if one is not fitted, dirt and moisture can enter the inside of the valve stem, potentially jamming or contaminating the sealing surfaces and causing a leak.
Metal, and some higher quality hard-plastic valve stem caps, often have a rubber washer or seal inside to help make an airtight seal. This type of cap protects from dirt and moisture and also helps prevent air from escaping from a slightly leaking Schrader valve. In addition, the rubber washer or seal prevents the cap from loosening and falling off due to vibrations by acting as a cushion between the cap and the valve stem. Most caps, however, are a softer plastic designed for protection only and are not equipped with a separate seal.
Now where might that little valve stem cap have gone? What probably happened was the last time you took your vehicle in for service your tire pressure may have been checked, but the valve stem cap may not have been put back on. It is also possible that the vibrations created by the spinning of your tires over thousands of miles caused the cap(s) to become loose and fall off.
If your vehicle has a tire pressure monitoring system (TPMS) it is even more important to have a valve stem cap to keep the moisture and dirt out. When the time comes to put your snow tires on, or to replace your tires, you may have to replace your tire pressure monitor sensor if the valve core has seized into the stem, possibly making your wallet an extra $60.00-$80.00 lighter than you expected!
If you take your vehicle in for a routine service that includes checking the tire air pressure, the valve stem caps should be properly tightened, and should be replaced if missing. If you find a mechanic who does this, you have found a keeper! Any mechanic that flags this issue as part of a used vehicle inspection is one who understands how important it is to be thorough even when it comes to seemingly insignificant things.