P0420 code is Catalyst System Efficiency Below Threshold. I hope the mechanic told you what this actually means. It doesn\'t necessarily mean that the catalytic converter is \"bad\" - it just means the engine computer determined that the voltage reading from the rear (downstream) O2 sensor is too similar to the front (upstream) O2 sensor. It could mean a bad cat, or it could indicate a bad rear O2 sensor.
I\'d say that 99% of the shops out there will blame the catalytic converter but in probably 70-80% of those cases, the catalytic converter is actually OK. Most commonly, it is the rear O2 sensor that is bad or you have an exhaust leak in front of the catalytic converter. in the case of the 8th generation Corolla - it is most likely a sensor glitch (rear O2 sensor or ECT sensor) or confused ECM.
Some more information would help - miles on the car and if it is one of the oil burners (tends to consume a considerable amount of oil between oil changes). If the car has more than 100,000 miles and uses more than a quart of oil every 1000 miles - then you probably have either a bad rear O2 sensor or bad catalytic converter. Possible that the front O2 sensor is bad - but that will flag a different OBD code.