Engine warning light went off?

I thought the engine warning light had to be turned off by something like an OBD. Today, after being lit for 7 days, mine turned it self off. The funny thing is I was at the Renault dealer where I bought it booking it in to have the codes read and the light turned off (it is still under guarantee). When I returned to my car after booking it in the light went out as normal instead of staying on after there's been a fault. Is there a time limit on the warning light or does it go off after the fault has cleared. I thought it stayed on until manually cleared. I think the codes will be stored so they can still read them next week, correct?

The first time the light came on it showed a cat fault, but as it freely revs to 6500 I don't think this can be the problem or am I wrong on this? I think it is more likely to be a bad connection on the Lambda. The rear Lambda, after the cat, is a new Bosch one. The front Lambda is very corroded, so maybe the connector is has oxidized too?
Oh, and the rear Lambda has a very very slight leak of exhaust gasses where it it bolted into the pipe, this wouldn't affect it, would it?

I have ordered two OBD tools, one with a cable which tells you what the fault is and where it is and another cheap WiFi one (for my Android phone) to keep in the car for emergencies. So in future I can read and clear the codes myself.

Engine warning light went off?