How do you diagnose a gas cap?
Cap does not tighten properly. One of the most common symptoms of an issue with the gas cap is a cap that does not tighten properly.
Fuel smell from the vehicle.
Check Engine Light comes on.
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Is check engine light always serious?
When your car's check engine light illuminates on your dashboard, it's usually accompanied by a sinking feeling in the pit of your stomach. The light could be a minor issue, such as a faulty gas cap, or it could mean something more serious, such as a misfiring engine. via
What are the main causes of a check engine light?
Oxygen sensor (or O2 sensor)
Catalytic converter failure.
Spark plug or ignition coil failure.
Airflow sensor failure.
Replace the thermostat.
Replace exhaust gas recirculation (EGR) valve and clean all EGR ports.
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