Worse mileage after new plugs and o2 sensor

05 gen 2 Prius, 208k miles. 
 

it threw a check engine light that came up as something about o2 voltage and then went away.  I figured it was old enough that it deserved a new one even if the problem self-cured. 
 

also of note, usually I get 48 mpg with the car.  For the last few fill-ups it has been 51-53.  A little odd, but I was happy. The light came on after a few tanks of this higher mileage.  
 

I read that changing plugs at the same time as o2 sensor was a good idea as access was greatly improved due to removing the cowl for the sensor. 
 

I didn't recall ever changing the plugs but afterwards, I realized that I had changed them at about 160k miles. 
 

so I got a set of factory plugs and a denso factory o2 sensor.  Removing the sensor was a pain (due to the heatshield) but that's for another thread. 
 

Well, my mileage instantly dropped to 43 mpg, same driving.  
 

I haven't checked for codes to see if there is any residual from the cel before but don't have any cel.
 

wondering if anyone can come up with a theory (and solution) for why my mileage has plummeted???  
 

when I get a chance I'm going to disconnect the 12v battery to reset monitors and clear codes in case there is something going on that was triggered by a previous problem but I'm not optimistic

I'm not sure what the procedure is, but have you tried a hard reset?  The computers in these are pretty smart and they have adapted to your driving style over the years.  Some changes to the fuel/air/spark situation may have it "hunting" for the right tune/map.

My guess is that it will re-learn pretty quickly, but it wouldn't hurt to pull the negative terminal and turn the headlight switch on overnight to see if things change.

What 02 did you get? I bought a set of cheap ones last summer and my car ran like ass. Swapped in a more expensive denso set and it ran well again.

Small vacuum leak that the ecu is compensating for?  

The old O2 may have drifted to read richer than normal.  This usually causes a catalyst code too, though, since the converter doesn't work very well when the engine is off stoich.  Maybe it didn't run failed for long enough for the converter monitors to run and fail.  (Gotta love checking mode 6 data)

Is this one of the engines that requires specific spark plugs per cylinder to be indexed correctly, or am I thinking of the Insight?

I'm not sure if your car is new enough, but most newer cars keep the adaptives in non volatile memory, so disconnecting the battery may not work.

It was, to the best of my knowledge, an authentic denso sensor. 
 

I'll poke around for a vacuum leak but access is pretty open so I'm not thinking that's likely.  But it's something to look for. 
 

I'll unplug the battery but not optimistic it will change anything.   
 

I'm tempted to put the old one back in but I had to dremel part of the shield off and those things hate vibration so it might be a fool's errand as there's a good chance if it was even partially good before, it's probably knackered now. 
 

ugh

Did you check the gap?  Had all sorts of problems with 2 cars this year when I didn't check the gap and it was off.  One would think after the first time I would have learned.

Front or rear sensor?  

Although, I have see that some sensors are offset than others.  No idea why.