Here's why:
* Gas Cap: A loose or faulty gas cap allows fuel vapors to escape. The system detects this, triggering the check engine light. Once the fuel level drops, the pressure within the tank might equalize, turning the light off.
* EVAP System: This system prevents fuel vapors from escaping into the atmosphere. A leak anywhere in the system (hoses, sensors, canister) can cause the same issue. The pressure change as the tank fills and then empties likely impacts the leak detection within the EVAP system.
What to do:
1. Check the gas cap: Make sure it's tightly sealed. If it's damaged, replace it. This is the cheapest and easiest fix to try first. A new gas cap is inexpensive.
2. Reset the check engine light: After tightening the gas cap, drive the car for a few days and see if the light comes back on. If the light remains off, the gas cap was likely the issue. If it returns, proceed to step 3. (Note: Many auto parts stores have free OBD-II scanners that can read and clear codes. If you have one, note the code before clearing the light. This can help identify the exact problem.)
3. Have the EVAP system checked: If replacing the gas cap doesn't solve the problem, you'll need to have a mechanic or diagnostic tool check the EVAP system for leaks. This might involve pressure testing various parts of the system to find the source of the leak. Repairing or replacing components of the EVAP system will be more expensive.
Ignoring the check engine light isn't advisable, as it could indicate a more serious underlying problem down the line. Addressing it promptly is recommended.
