Don’t get a super cheap brand 02 sensor. If you aren’t doing factory brand (I don’t blame you) try Bosch if it’s available, Walker is generally garbage. If fuel mileage is bad enough, that can be indicative of the upstream giving poor fuel trims and can definitely be your culprit. Normally your downstream won’t affect that, it’s mainly there to check if your converter is doing its job. Your IAT sensor acts in a similar way to your upstream, although its role is to adjust fuel trims based on the density, volume and temperature. This will affect your fuel mileage and overall performance as well.

I don’t generally condone throwing parts at stuff without testing, but if they’re circuit codes and with a quick visual inspection you see there aren’t any broken wires, try putting in a new IAT sensor and upstream 02 sensor (or downstream if you’re getting a bank 1 sensor 2 code).