What is triggering my p0420 code?

Air conditioner is adding a continuous load on the engine which can alter idle speed as well as various operating system conditions - that may be the relationship between the a/c being engaged and the light turning on as this would certainly have an effect on exhaust gases .. Will be more pronounced if another operating system problem exists. There is a slim possibility the downstream 02 sensor is causing this problem - however if scan tool readings reveal upstream and downstream sensors are switching simultaneously and showing the same values with engine on - catalyst HAS been depleted.. Some code readers have the capability of live sensor data readings. Good luck!