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P0172 is a rich code, then P0420 is basically saying the catalytic converter isn't working well. Once the codes are cleared, the P0420 code takes around 30-50 miles to come back if it's still a problem. That's one my car has currently but it doesn't effect much which means the cat on my car isn't plugging up (oil build up most likely would be the cause for that).
MAF makes sense for the P0172 though, it must have been detecting more air going into the ending than it really was getting, so dumped too much fuel. Makes sense why econ dropped and such.
MAF sensors can also be cleaned with a special cleaner sold at parts stores. A dirty MAF can cause similar issues and I'd be checking the air filter and gasket for the air filter cover. MAF's can be really touchy sometimes. Either case replacing it isn't a bad option, big time if they are somewhat cheap.
The car might have to "relearn" how to run again, so the first 10-20 miles it might not run perfectly. Probably not enough of an effect to be a problem with your testing, but it's something to be mindful of. Sounds like it was a sudden change, so I'd guess the MAF went bad.
Hopefully solid increase in the next tank over the last.
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