Upstream O2 sensor is for the air fuel ratio. Down stream O2 is measuring the same thing but for emissions this time. Yes, this is usually constant but not perfect as stats change and cat converter gets saturated and adjustments made. If it fluctuates a lot, then it could indicate a failing cat converter. Also you won't have many symptoms like you would with the upstream o2 sensor
How have you been making out with this?
The downstream failed on me and caused a 7mpg lower average. The upstream failed and caused the same thing.
Btw OP if you replace the O2 get an a.c. Delco one and save yourself a bunch of grief. Sometimes when they start failing you need to look at the freeze frame data to see. Also there's tests most professional scanners have that test the reaction time of the O2 and if it's to slow it fails
2009 Ford Escape Limited - 3.0L, 6 speed auto, 4x4, 25/33 city/hwy imp mpg.
2017 F150 XLT-XTR - 5.0L, 4x4, 3.55's, E-locker, xtr chrome package, fx4 off-road package.
20/25 imp mpg