I checked and: It could be an exhaust system leak, a malfunctioning 02 sensor, bad wiring to an 02 sensor, or a bad catalytic converter.
Get under your car and check for possible exhaust leaks.
Check your wires under your car that run between the underbody and the O2 sensors.
Just like we spray the MAF with Disc Brake Cleaner, Carburetor/Throttle Body Cleaner (even though we don't have a carburetor) etc, you can try spraying the O2 sensors(but make sure you won't harm the sensors with the spray chemical), but first consider:
How many miles do you have on your '00 Celica? And is that the original stock CAT on the car (assuming of course that you are the original owner of the car and not another person who used an aftermarket and put the stock back on and then sold it!). Hey, I just think of as many possibilities as I can 'cause I can't help it.