Rep Power: 0

I recently had my alternator replace, got the car back and its running great, 2 days ago, i was driving put on the ac and the check engine light came on, car was still running great. I took the car to Autozone and it came back po420 (P0420 Catalyst System Efficiency Below Threshold (Bank 1)).
-bad A/f ration
-something with cat
-bad downstream 02
-air leak

So I checked everything out, the exhaust looks fine, didn't see any holes, tightened up my air intake and then reset the light. Today I was driving the car and decided to put the ac on, the check engine light was off for 2 days and this was the first time I was using the AC since the light came on a few days back. Same thing happened, the check engine light came on within a minute of turning on the AC.



any ideas the car runs fine with ac on or off, just weird the ac makes it come on.