Before condemning your Toyota Corolla's catalytic converter we'd recommend checking two things closely. One is engine cylinder compression, and the second is air/fuel ratio.
Your Toyota has quite a few miles on it and over the years probably built up a good amount of carbon on the piston and valve heads. Carbon build up leads to high cylinder compression and high cylinder compression leads to high engine temperatures. Since NO is created when engine temperature rise above 2500f, on vehicles with high mileage, carbon build up and high cylinder compression should not be ignored when diagnosing NOx problems.
The second thing to make sure is not causing your Toyota Corolla to produce high NO is a lean fuel mixture. A lean fuel mixture is when the engine receives less fuel then is necessary to obtain efficient combustion. Lean fuel mixtures may be due to vacuum leaks or defective fuel metering components. Vacuum leaks will allow the suction of additional and un-metered air into the combustion mixture or exhaust (depending on where the vacuum leak is located) disturbing pre/post fuel combustion and increasing NOx emissions.
We recommend replacing the catalytic converter only after all other emission systems have been diagnosed and no faults found.
