Mysterious engine warning light issues?

[Lofarus]

I've had my Lanos for 7 months now, still running strong and smooth. But the engine warning light is constantly on for some reason. It could be many reasons, it started to come on since I re threaded a bolt on my exhaust manifold to stop it from being so noisy, could the oxygen sensors suddenly go haywire because of something like that? When I was doing that I noticed that 2 of my ignition leads were dodgy so I replaced them with some universal ones for the time being, I know they are not standard Daewoo leads so could that be the case? I have tried resetting the ECU, it worked for a couple of days and came back on again after a long ish run. Needless to say I am a little confused at this point. I also have a blowing exhaust at the moment, would that be causing it? It wouldn't bother me so much normally because the car still runs as it should, but I have the MOT coming up early in the next year and I don't want it failing over something so small :S Any advice will be appreciated

[daewooluvr]

Yes I believe the exhaust leak would cause it but be sure to get the ODBII Scanned to confirm.

[Puddle31]

take it somewhere to get the code (s) read and see what the problem is.