Gas Cap
A threaded cap that seals the fuel tank filler neck, preventing fuel vapor leaks and playing a key role in the EVAP system.
The gas cap, or fuel filler cap, is a simple but essential component of a vehicle's emissions control system. While its primary visible purpose is to prevent dirt and water from entering the fuel tank, it also acts as a critical seal for the fuel tank, keeping harmful gasoline vapors from evaporating into the atmosphere.
Modern gas caps contain a built-in pressure relief valve and a rubber gasket. When the cap is screwed onto the filler neck, it seals the fuel tank to allow the evaporative emissions (EVAP) system to maintain pressure. If the rubber seal on the gas cap dry-rots, cracks, or if the cap is left loose, the EVAP system will detect a pressure leak during its self-test.
A loose or leaking gas cap is one of the most common causes of a check engine light illumination (typically triggering code P0456 or P0457 for a small EVAP leak). If your check engine light comes on shortly after refueling, checking and tightening the gas cap is the first step. If the cap is cracked or has a damaged seal, replacing it with a new one is a cheap and easy fix that will turn off the warning light after a few driving cycles.