Lots of codes
 

I have an 2007 suburban LTZ 5.3 with 140,000 miles. Earlier this year I got a CEL for P0420 and P0430 catalyst b # 1 and 2. So I took the car to the dealer they quoted me for the cats to be replaced. I had them replaced and the codes kept popping up. Now after spending a lot of money at the dealer and various other shops these are the codes I have P0121 TPS, P0496 evap emission system flow, P0172 and 0175 system rich in bank 1 and 2, P0420 and 0430 for the cats. I'm familiar with these codes but where do I start? With the evap purge valve or the throttle body? The gas cap is new and the the throttle body is clean.

Welcome to the forum...
The Evap and throttle position are probably real, though I would focus on the TPS as the evap will not cause any drive-ability issues the TPS will....

As for the cat codes, I would have started with the O2 sensors before assuming the cats were bad considering the cost...

Thanks for the response and I've been trying to get this car through emission since August. . But I'm going to get working on the TPS in the morning. Then o2 sensors.