1992 Chevy suburban check engine light not on. Code 12. What do I need to fix?

What is your question?

Now I'm not sure if it's a situation but my check engine light does not come on although it does work because I've seen it on before. But that was a different problem which has already been taken care of. The truck drives fine so it seems to me but as I was reading it says that the truck could run fine and seem like you was nothing wrong with it but the code number 12 came up which is to me odd because for some reason I thought it was going to be 13 being the O2 sensor. Anyways I was just wondering

What makes this problem better or worse?

From Reading I just feel like the engine should run better than what it does now because the check engine light doesn't come on.

How long has your 1992 Chevrolet C1500 Suburban had this problem?

I just checked up the code the other day and it came up number 12 and it says something about the distributor but does that mean that I need to change my distributor cap and rotor