When we arrive at a flat tire call, the first thing the technician does is find the leak. Soap-and-water on a partially inflated tire bubbles at the leak point. The location determines the fix.
Tread punctures (the flat surface that touches the road) from nails, screws, or sharp gravel can usually be plug-sealed on-site with a vulcanized plug kit. We clean the puncture, ream it slightly to seat the plug, insert. With proper installation, a tread plug holds for the remaining service life of the tire.
Sidewall punctures (the vertical surface facing outward) cannot be safely plugged. The sidewall flexes constantly while driving — any plug will eventually fail and you'll have a high-speed blowout. We mount the spare and refer you to a tire shop.
If the tire was driven on flat for more than a mile or two, we recommend not plug-sealing even tread punctures. Driving flat damages the sidewall structure internally even if it looks fine externally.
Spare tire pressure matters. Spares often sit at low pressure in the trunk for years. Always check the spare's pressure annually.