check engine light?

95 tsi, my check engine light just came on 2 weeks ago. the car has always blow black smoke out the pipe and the a/f gauge reads lean all the time, i know there not very accurate. but could this be that my 02 finally went bad? or could it be something else?
p.s. i unhooked the battery and the light went off for like 5 days or so. if this was very serious wouldnt it come on right away?
thanks
tyler

Well 3 things:

1) When you get a CEL, you should get your codes read...it is signifying a problem...making the light go away, doesn't mean your problem is solved. So when you get a CEL, go to autozone and have them pull the diagnostic codes for free as it will help identify the underlying problem

2) if you have black smoke, it is from unburnt fuel in the cylinder...hence you are running rich

3) if you are running that rich, and your getting really lean readings, it could be your o2 sensors, could be a burnt out cat...could be a few things...this is why checking the code is important.

thanks, oh i dont have a cat anymore.