Trying to diagnose reduced fuel mileage and O2 sensor data

[Scopeman]

I am experiencing reduced fuel mileage, but no other apparent symptoms or codes set. The car seems to run fine, as usual, but with noticeably increased fuel consumption by about 10-12 %. I don't think this is due to weather, winter fuel, etc.

In my exploration of this, I have graphed, using VIDA, oxygen sensor, short and long term fuel trim data, and engine RPM, see the attached graph.
Something I notice, and do not understand, is that the post-cat O2 sensor varies pretty much all the time. My understanding of this function is that this output voltage should remain fairly constant under reasonably constant load conditions, at a voltage about .7V, indicating an oxygen rich condition on the outlet of the catalytic converter. This does not appear to be the case.

The red trace data peaks are the result of lightly blipping the throttle.

Ideas?

[pbierre]

I agree that the downstream O2 signal looks quite abnormal. Surprised the ECM isn't throwing a code.

[Scopeman]

The idle RPM seems normal, and about 650. No codes now or ever...
Also, I have a Blue Driver OBD scanner, and when connected here, the data looks similar, but it indicated LTFT in the range of about 7-9, varying quite a bit.
I don't know why the VIDA number is different.

[pbierre]

You're inducing small RPMs increase, and seeing "thrashing" at the downstream O2 sensor.
I suggest to post your signal traces on a site like this, where O2 sensors are discussed:

https://www.fixdapp.com/blog/oxygen-sensor

[Scopeman]

Belated Update:

I replaced both the upstream wideband A/F sensor, and the downstream in-cat O2 sensor with Bosch OE replacements. This seems to have cured the problem, resulting in both an increase in fuel mileage and better throttle response.
At 140K miles I think that the original sensors were poor, but not enough for the ECU to flag.
I have had a similar experience, at a similar mileage on another vehicle.

[Waterdog]

If your mileage is still not up to where it used to be, you might consider getting your injectors cleaned. I saw a sharp drop in mileage a few thousand miles ago, but sending the injectors out to be cleaned brought the mileage back up to where it used to be. The car is at 188K now. I had replaced both of the O2 sensors at about 166K and also changed out the coil packs and plugs about 172K. About the only thing left that might have been causing the mileage drop was the injectors - and they were indeed the issue.