ok so first i will tell you my story i had fixed this a couple of months ago. o2 sensors and such. i have been running the same set up for months now. so here it is i have ebay headers going into a stock cat and now it is throwing code 45 looked at all the bolts tight as a button and even took off the neg battery terminal. so what do you think it could be??
get back to me soon i hate yellow lights

 

What year/OBD is your Civic?

 

oh yeah sorry about that its a 2000 ex obdII

 

ttt

 

Do you have anything modified as far as the fuel delivery system?

 

Try running a different octane gas, check the gap on your plugs, make sure your injectors are all measuring the correct amount of resistance, make sure your air filter is clean, not obstructed. You aren't running a VTEC hack or anything like that are you?

EDIT: did you replace both O2 sensors?