Is there any way to test to see if the oxygen sensor is working correctly?

I just put in the upstream and downstream sensors, and reset the code, but a couple days later, the code came back on. I double checked to make sure the electrical connection had not worked itself loose, but seemed ok.

The auto parts store checked the code, and was the upstream.

O2 sensors put out a 0-1V signal, wide band O2 is 0-5V.

What is the code description? High, Low or invalid signal?

Other engine conditions can make a high or low signal appear.

The guy at the auto parts store just told me it was bank 1, which I guess is the upstream/front sensor.

You need a real scanner or a proper code number to cross reference. The term "bank 1" only tells you a location.

You need the code# to determine the fault.

Yes.
With an ohm meter it kinda works.
Technically an oscilloscope

The fluctuation can be measured.

You probably need a cat.
What year, how many miles, any contamination (oil burn or leaded fuel or sea foam use?)

96 VW Jetta. Used Sea Foam. I replaced the Cat about 5 years ago.

So, it may not even be the new sensors at this point?

After market cats are often very short lifer. What brand did you use. Some of te cheaper brands have bee reported to have 12 month lifespans

I don't recall.

How does one know it is the cat, then, and not the sensors?

By testing the sensors for laziness With an oscilloscope

Thanks.

I'll have my electrician take a look at it for me..?

Pro auto shops will have a scope or be familiar enough to use an ohm

I had O'Reilleys reset the code, and it was PO131, Circuit-low voltage.

So far the light has not come back one since noon. I did take the connectors off, and hose them down with some MAF sensor cleaner, and pushed them back on hard, as maybe that could have been part of the problem, or not making good contact, I wonder??