Trying to wrap my head around fault code P0430 and P0420

[mump]

I'd put money on the MAF sensor. This summer I got the P0430 code, took it to the Porsche dealer since it was real close when I lved in Germany. They cleared it and said no big deal unless it comes back. 2 days later, P0420. Cleared it. 2 days later, P0430. It did this 5-6 times. I went back to the dealer requesting my O2 sensors changed. They said I would be wasting money and told me to get a new MAF sensor. Ordered it from Rock Auto since it was cheaper. Never got the code since.