Ok, maybe you can do the diagnosis that they couldn't, because we can arm you with more info.
Here'* the GM FSM page for P0102, Maf low frequency. It mentions vacuum leaks, etc that could cause the code.
Here'* the P0420 Cat low efficiency code, I wasn't sure as you didn't mention low, but it'* the most common. The interesting thing here is how P0102 can't exist for this code to be set. Which means you are seeing a questionable cat code before the maf. The cat issue may be causing the maf code.