Steady versus flashing
This distinction is the single most useful thing to know about the light. Steady means a fault is stored and the car is running within limits it can manage. Flashing means raw unburnt fuel is being dumped into the exhaust right now, and the converter is being cooked. The repair for the second one gets more expensive by the mile.
Why the free code read at the parts store isn't enough
The scan gives you a code such as P0171 — system too lean. That is a symptom, not a part. It could be a vacuum leak, a dirty mass airflow sensor, a failing fuel pump, or a leaking injector. Four repairs, four prices. Buying parts against the code alone is guessing, and it is why people end up replacing three things before finding the fourth.
The Gwinnett County angle
Georgia's emissions programme reads your car's computer rather than the tailpipe on most vehicles. A lit light is an automatic fail, and so is a car with incomplete readiness monitors — which is exactly what you get if you clear the code shortly before testing. See our emissions failure page.
What we would do
Pull every code plus the freeze-frame data showing conditions when the fault set. Check manufacturer service bulletins for known issues on your engine. Test the circuit with live data rather than swapping parts. Photograph the finding and quote it before touching anything.