emissions fail

hi has any1 had problem with emissions going very high on idle.Failed nct in my sti last week,have full system on it and when i went to fit cat it wouldnt match up with the system i have on it,must get original system somewhere but even the cat fitted wouldnt fix my problem.on low idle the co reading is at 0.70 which is close to passing but after a while it jumps crazy high up to 6.11 and stays there for a few seconds then goes back down to 0.70,it keeps doing this,have standard ecu back in it ,Im thinking oxygen sensor or air flow meter,but air flow meter is newish but not main dealer.What do ye guys think?

Get it hooked up to a computer for diagnostics, a few traders here can do it. They should be able to tell if a sensor is gone etc.

Steve.