didn't pass smog...running rich

[heated heavy]

my 89 240sx didnt pass smog today because of a really high percentage in HC and CO which i was told because of my car running rich.
what are each and every possibility of running rich? i want to narrow the problem down by process of elimination. so if you have any links from this forum or anywhere else that you can give me to help the situation i would very much appreciate it. flames welcomed because im sure this has been brought up many times but i coulnt find any exact details on running rich on this forum.

[mistaanime]

hehe any engine work?

[DuffMan]

Could just be an old worn out cat. I failed on HC and CO, but I get the gas milage the car is rated at. I replaced the cat and passed. What kind of milage do you get? A new cat might be the way to go.

[old_s13]

cat and o2 sensor are the keys to passing smog.

egr and aiv are the keys to passing smog AT IDLE.

[heated heavy]

ya, i was thinking a new cat and o2 sensor would be it. how much for a new cat and a mechanic to weld it on? where can i get a new cat? i dont have a service manual so where exactly is the o2 sensor located?
thanks