Here's how it works:
1. Sealed System: The EVAP system is designed to prevent fuel vapors from escaping into the atmosphere. It does this by sealing the fuel system, including the fuel tank, fuel lines, and the fuel cap.
2. Pressure Sensor: A pressure sensor monitors the pressure inside the fuel tank. A properly sealed system will maintain a specific pressure range.
3. Loose Cap Detection: A loose or faulty gas cap allows fuel vapors to escape, causing a pressure drop. The pressure sensor detects this drop.
4. Check Engine Light Trigger: The drop in pressure triggers the "check fuel cap" light (often part of the larger "check engine" light system or a separate indicator). This alerts the driver to a potential leak in the EVAP system, often starting with a loose or damaged gas cap.
5. OBD-II Codes: Beyond the light, the car's onboard diagnostics (OBD-II) will store a trouble code related to the EVAP system if the pressure remains low for an extended period. A mechanic can read these codes to pinpoint the problem more precisely. It might be a faulty gas cap, a leak in the fuel system, or a problem with the EVAP system itself.
In short: The light doesn't actively "check" the cap, but it reacts to the pressure change caused by a leak that a loose or damaged cap is likely to cause. Tightening the gas cap is the first step in troubleshooting. If the light remains on after tightening the cap, further diagnostics are necessary.
