High CO implies the possibility of an overly rich mixture, preventing complete combustion
This is what is suppsed to happen
CH2 + O2 = CO2 + H20
This is what is happening on your car
CH2 + O2 = CO2 + CO + H20
So either too much gasoline, or too little air. Checking fuel pressure and replacing engine air filter seems like a good idea. Correct equation requires a lot of heat too, so double check coolant is correct operating temp. If not, replace thermostat or whatever is causing coolant temp to be too low.
I’ve had a too much HC problem on my Corolla before, not the same as too much CO, but this might be helpful for ideas anyway. Is your emissions testing done parked at idle, or on a car treadmill at speed? Suggest to not assume it is cat, could well be something easier to fix. Tell us how your 1.6L A3 engine is configured? Model year? Distributor or coil on plug?