Two sorry shops took you down the wrong road. Unfortunately, those types of shops/mechanics are far too many.
The code P0430 is for out of range voltage values of sensor signals in the catalytic converter system, bank 2. This is on the other side of the engine from bank 1 (which has #1 spark plug, and would have code P0420 if there were a problem indicated in that circuit).
This is the type of troubleshooting checklist a good mechanic follows for Codes P0420, and P0430 (and, s/he doesn’t pretend to follow such a checklist): http://www.obd-codes.com/p0420