You have done many things including having just changed plugs, which means you are very hands on....
So I apologize if some of my ideas are dopey,
I am inquiring into your diagnostic journey.
My wife drives a 2012 AWD, and she now is getting 16.3 mpg, up from 15.1 about 2 months ago.... I had her intake valves walnut blasted.....at about 79k miles. (And new plugs since we were in there already)
Her average trip is 10 minutes, stop and go driving in the "burbs" with the occasional 45 minute highway blast to go to a shopping mall or visit our kids......
So I concur, your milage is low.
My 2 cents
Are you using top tier gasoline? ( top tier, includes regular gasoline...it does not imply mid grade or premium.) if not, only use only top tier.
Regardless, ^^ , try a few tanks of shell premium, or chevron premium..... it will clean carbon from fuel injectors and clean any carbon build up on upper piston ring. I know it's an extra 80 cents per gallon , and definitely not worth it on an mpg basis, I suggest it for its cleaning of carbon benefits
also before your next oil change, run a tankful of Techron fuel system treatment, it has PEA, and is similar to what GM sells at dealership, and it is good for the plastic parts of your fuel system, will clean fuel injectors and will clean carbon off upper piston ring...
You need to change your engine oil after you do this... piston blow by and the PEA, will contaminate the engine oil....
then make sure you use dexos approved 100% synthetic engine oil.
It should go without saying ... but .. you need to check engine oil levels every 1000 miles, then more often when oil consumption is detected, after all all gdi (gas direct injection) engines will eventually burn oil.
When was the last time you cleaned your throttle body?
I know you were looking at it when you changed your plugs...
Lots of crankcase gunk gets into your air box ( between air filter and throttle body) since the clean side PCV has reverse flow under wide open throttle., which deposits crankcase gunk into airbox.
inspect the air box and clean the crankcase goop, I am sure you did this, since you had to remove intake manifold to change plugs.... was your air box oily?
And by the way, how were your spark plugs?
Did you look at your intake valves ? While you were changing your plugs?
Tire pressure? Make sure your tires are pumped up to 36 psi, don't rely on the cars tire pressure monitor for accuracy, buy the $30 Michelin tire pressure gauge, accurate to 0.1 psi.....
And finally, since you are asking about O2 sensors,
do you have the $70 OBDlink mx+ Bluetooth device that you can pair with your iphone?
You can see o2 sensor voltages as you drive,
fuel trims, engine temperature and other parameters,
that might help find root cause of lower mileage......
In addition to seeing all gm codes, . ,,,,, if any are thrown.
I have one ^^ , if you wanted to know about any specific parameter, to compare your parameter values, I could let you know what I am getting on the 2012 traverse....
2012 Traverse AWD (80k miles) with Elite E2 X catch can
Three Equinox AWD, 2013, 2016, and 2016 (all are 2.4 liter, 4 cylinder engines)
2017 Equinox AWD, 6 cylinder engine, with Elite E2 X catch can