If the original code was something like P0131, which officially means there is a low voltage coming from the oxygen sensor, most "technicians" will just read that code and say the sensor needs replaced. In reality what the ECU is actually saying is, over the last 50 or so miles, the reading I have seen from the oxygen sensor is too low, this can be caused by almost anything from a blocked/burst hose somewhere in the engine bay to a leaking exhaust (does it sound noisy?) if you just change the sensor without checking it properly (and that doesnt get done by a code reader by the way) and then just clear the fault code, it will keep the light out, making you think you have sorted it, but the cars brain tests it all the time, but wont tell you its failed, put the light on, until its finished its test, and that can take up to 100km (60miles).
What I am trying to get accross is, the light is on because the brain has seen something it doesnt trust, like or believe. Depending on the fault, the light MIGHT go out on its own, but that is unlikely and in any case it is unwise to ignore as it will most probably end up costing you money either on extra fuel or in an even bigger repair bill at some point.
There is no fancy computers/codes readers/scanners on the market that "fix" cars (if I ever see one I am buying all of them, they will be worth a fortune) only ones that can help properly trained people have a starting point in their diagnosis and assist them with the dreaded intermittant fault that never seems to happen within 200 miles of a workshop!
My advice is to get it checked by someone who knows what they are doing
, and not someone with a glorified Gameboy and a set of spanners. Nothing hacks me off more than people getting ripped off, and giving us grunts a bad name.
I will step off my soapbox now!