That light is on because something on the engine, probably emissions related is not working properly. I am not sure what the exact words were that your mechanic used, but it doesn’t seem like sound advice unless there were other conditions involved. $500 nowadays does not go very far in repairs, so it sounds like a relatively straightforward fix, but I agree, have the codes read for free and get a 2nd opinion. The stress builds up when after $500 the light comes back on…