my 1993 honda prelude will not pass e-check emissions test due to high nitroge/oxygen compounds. what causes this failure and how much does ir cost to repair?

Excessive carbon build-up in combustion chamber causing higher than normal combustion temp. , system running hotter than designed , catalytic converter not efficient enough. --some of the possibilities.Without PROPER analysis and diagnosis , cannot begin to guess on $$ could be as simple as running a few tanks of premium (93)octane gas through it................