HELP.. exhaust smells bad

Sup pplz.

i have a 94 civic dx with a super bad smell coming out the exhaust. a friend of mine told me the smell is like unburned fuel which i believe would explain my bad gas milage too.

i hooked up an air/fuel ration to it (at the o2 sensor) and the guage stays stuck on rich.

they car only recently started doing this.. i have also noticed that sometimes the check engine light goes on.. but when i turn off the car and turn it back off its gone, so i cant get an ecu reading.

what would cause this.. and what can i do to fix it.
is it the o2 sensor?


---- specs------
94 civic dx
sohc non-vtec engine
ractive headers
tenzo catback exhaust