When you erase the fault codes it turns the light off. If the light comes back on then the fault has re-occurred.

IF you install an elbow, and IF you reset the codes, and IF the elbow does what it should, then the light will stay off. If the light comes back on, then the elbow hasn't worked. My light is on all the time, I just ignore it now, and check the codes occasionally to make sure nothing else has gone wrong lol