Check Engine light

[KNJ]

Hi! In April, my Yukon was throwing codes P0172 P0175. Local GMC dealer in Louisiana "fixed", by replacing some sensor. Traveled to PA, codes came back on. Local GMC dealer fixed by replacing BCM, body control module in June. Now my check engine light is back on! Power train Warranty ran out 2-3 weeks ago. Also had extended warranty, which also ran out at 5 years, which was Sept. Code onstar says its throwing now is P0430, all of the codes from I can find are related to exhaust system. My question is are these 3 codes related??? Could this be something they will have to fix under warranty, given it's related and has been worked on several times? 

If not, what does this mean? Catalytic converter bad? Help! My husband is being laid off in the next week. So, obviously finances are tight. Yes, obviously itll be fixed regardless, just trying to plan and cut back elsewhere if needed. This has been a great vehicle! As luck has it, trouble starts when warranty runs out. 

 

Really wanting to trade, but waiting on new body

 

Adding mileage if matters, 91K

[nfisherman]

Before you have anything else fixed, fill it up a couple of times with the best top tier premium gas you can find. My Sierra did the same thing at 98K miles. I got the P0420 code. My dealer said that the converters needed to be replaced for $1,600. I read an article that said sometimes the catalytic converter gets fouled if you buy cheap gas. I bought cheap gas all the time. The article said that good premium gas can sometimes clean up the converter and the codes will go away. It worked for me and I only spent an extra $8 per tank on the premium gas. It doesn't cost much to try.