The primary function of the plug cap is to transfer the high voltage from the ignition coil to the spark plug, which ignites the fuel-air mixture in the engine. The plug cap ensures that the electrical connection is secure and prevents the loss of electrical energy that could cause misfires or poor engine performance. It is typically made of durable, heat-resistant materials, such as rubber or plastic, to withstand the high temperatures and electrical stresses generated during engine operation.

The plug cap also serves as a protective cover for the spark plug and surrounding components, preventing dirt, moisture, and other contaminants from entering the ignition system. It helps to maintain a sealed environment, preventing external elements that could cause corrosion, rust, or degradation of the electrical components, ensuring the longevity and reliability of the ignition system.