That is a catalyst inefficiency code. You need to have a good diagnostic test done to find out why. You could have a problem with the heated oxygen sensor, or the catalytic converter itself could just be not doing its job. If that's the case, you could have something else that has caused it to fail...so looking at it with a good diagnosis is in order.
P0420 is low catalytic converter efficiency and may require a new cat converter to repair. Sometimes a failing O2 sensor can cause the code. If it is a failed cat converter make sure what caused the failure is addressed.