Hello and thank you for using just answer.
The emission control system is strictly monitored and controlled by the ECU on this car.
If you had an issue with an o2 or air fuel ratio sensor you would have an additional code in the system. For example if you had a P1135 and a P0420 I would diagnose the other code and repair that first. When you have a stand alone P0420 code this means that the computer has passed the o2 sensor monitor and it is working as designed however the catalyst is not operating properly.
There is a service bulletin for this concern. Not ALL corollas can be reprogrammed, on some the engine control module must be REPLACED because it is designed by delco (general motors) and cannot be reflashed, on these it must be replaced.
Yes update the computer calibration (or replace) and also replace the converter.
I also highly recommend a spark plug replacement at the same time *car uses iridium spark plugs.
If you look under the passenger dash the computer is under the glove box above where your feet rest. If it has a Delco computer with a part # XXXXX computer then it must be REPLACED, all others can be reprogrammed with the dealers scan tool. Hope this clarifies. thanks.
I just replaced the plugs (NKG iridium) and air filter yesterday and the "light" came on after 30 miles highway driving.
On the ECU, would I have to replace it in addition to the converter?
Reread my last statement.
Yes the converter needs to be replaced.
The computer can be replaced or reprogrammed depending upon the information I posted above.
Thank you, XXXXX XXXXX good evening.
One last note here- converters are VERY expensive, check out www.metrotoyotapartsnow.com they sell online at wholesale prices, you may save some $$$$ here. Thanks.