You definitely have an exhaust leak somewhere in the engine compartment. It could be a cracked manifold, blown gasket, rusted out pipe, leaking catalytic converter case, etc. The reason that the P0420 code is present is that in addition to exhaust escaping, air is constantly being drawn in. The ECU will try to adjust the mixture to swing that rich but cannot. The pre and post O2 sensors will not toggle so the ECU cannot verify the efficiency of the catalyst. What the ECU wants is to drive the pre O2 to read rich followed after a short period by the post O2 sensor toggling rich. Then forcing the pre O2 sensor lean followed shortly by the post O2 sensor toggling lean.
Have a mechanic search out the exhaust leak; replace or repair the fault; reset the CEL; do a drive cycle that includes the catalyst efficiency check; and see if the code returns. If it does have the tracking of the two O2 sensors analysed. If they are tracking each other without delay, consider replacing the catalytic converter.
Good Luck on this.