A heat gun is a tool that blows hot air, similar to a hair dryer but operating at much higher temperatures, typically between 500 and 1000 degrees Fahrenheit. In automotive work, it is primarily used to shrink heat-shrink tubing over electrical connections. This tubing shrinks in diameter when heated, forming a tight, insulated seal that protects the copper wire splice from water, dirt, and road salt.
Using a lighter or match to shrink tubing is common, but it is dangerous and sloppy. Open flames can burn the plastic, melt the wire insulation, and create a fire hazard near fuel lines or oily engines. The heat gun provides a clean, controlled stream of air that shrinks the tubing evenly without burning it. It is also useful for softening plastic body trim, removing decals, and heating up rusted nuts to break their thread lock.