A code is a starting point, not an answer. When that light comes on, the computer has seen a reading outside the range it expects and stored a code for it. The code names the system involved. It doesn't name the broken part. One code can have five believable causes sitting behind it, and the only honest way to tell them apart is to test.
Steady or flashing matters. A steady light means the computer wants someone to look at it, and you can usually finish your errands and get it scheduled. A flashing light means the engine is misfiring right now and unburned fuel is heading into the exhaust. If it is flashing, pull over when it is safe and have the car towed or looked at before you drive it further.
Diagnosis starts with every stored code plus the freeze frame data that came with it, so we can see what the engine was doing at the moment the light triggered. Then we watch live data with the engine running. Fuel trims show whether the computer is adding or pulling fuel to cover for something. Sensor readings get compared against what the engine is actually doing. From there our ASE certified techs test the specific suspects, which is how a failing sensor gets separated from a vacuum leak or a wiring problem instead of replacing parts in order until the light goes out.
Some codes have nothing to do with how the car drives. Evaporative emissions codes are the usual example: the car feels perfectly normal while fuel vapor quietly leaks away. Sometimes it really is a loose or worn gas cap, and we'll tell you that. Often the cap is fine and a smoke test finds the leak somewhere else in the fuel vapor system.
You leave knowing the actual cause, with inspection photos on your phone and a written quote. Whether you fix it that day is a separate decision.
