The check engine light is an important warning indicator on a vehicle's dashboard that signals a potential issue with the engine or emissions system. When the check engine light comes on, it means that the vehicle's onboard computer system, known as the OBD-II (On-Board Diagnostics), has detected a fault or error code. In the state of Virginia, an illuminated check engine light will likely fail an emissions test. Some common problems that can trigger the check engine light include:

1. Faulty oxygen sensor: The oxygen sensor measures the amount of oxygen in the exhaust system and helps regulate the air-fuel mixture in the engine. A faulty sensor can cause the engine to run rich or lean, which can lead to decreased fuel efficiency and increased emissions.
2. Loose or damaged gas cap: A loose or damaged gas cap can cause fuel vapors to leak out of the gas tank, which can trigger the check engine light.
3. Malfunctioning catalytic converter: The catalytic converter is responsible for reducing harmful emissions from the engine. If it fails, it can cause the check engine light to come on and result in increased emissions.
4. Failed spark plugs or ignition coils: The spark plugs and ignition coils are responsible for igniting the fuel in the engine. If they fail, the engine may misfire or run rough, which can cause the check engine light to come on.
5. Failed mass airflow sensor: The mass airflow sensor measures the amount of air entering the engine and helps regulate the air-fuel mixture. If it fails, it can cause the engine to run poorly or not start at all.

When the check engine light comes on, it's important to have the vehicle inspected and diagnosed by a qualified technician. The technician will use a diagnostic tool to read the error code stored in the vehicle's onboard computer and identify the specific problem. Depending on the issue, the mechanic may need to perform repairs such as:

1. Replacing faulty sensors: If a faulty sensor is identified as the cause of the problem, it may need to be replaced.
2. Repairing or replacing damaged components: If a component such as the catalytic converter or ignition coil is damaged or not functioning properly, it may need to be repaired or replaced.
3. Clearing error codes: Once the problem has been identified and repaired, the mechanic will clear the error codes and reset the check engine light.

It's important to address check engine light issues promptly to prevent further damage to the engine or emissions system. Regular vehicle maintenance can also help prevent problems and ensure that the vehicle is running smoothly.