Oxygen Sensor Diagnostics and Specific complaint.

First, I am viewing the oxygen sensor outputs via an OBDII interface connected to a laptop PC. For the upstream sensors, I see a waveform over time that is switching from high to low and back high, as expected. The short term fuel trim is also averaging around zero. I am using the tool from:
http://www.obddiagnostics.com/
If you follow the Windows Screenshot link, you can see a representative screenshot for this test at the next to last one, just before the one with the analog gauges. Here is the link:
http://www.obddiagnostics.com/ScrnShot/Winscreen.html
My upstream sensors pretty much look like this.

My questions are these: should the downstream sensor waveform also be switching? Also, what should the down stream fuel trim read?

I am getting only sporadic switching with the red waveform hovering around the middle. The blue line indicating the fuel trim is at 100% at the top of the display.

Second, this pertains to a 2001 Toyota Corolla but I am posting it here because I find that there seems to be more detailed knowledge in this forum. The other symptoms are:
1) higly intermittent CEL light - on once every two weeks and goes out after a day.
2) P0171 fault code stored on CEL event.
3) usually comes on when cruising at 60mph on highway and gas tank at half full.
4) Sometimes hard to start. Turns over nicely but coughs and sputters to life.
5) Sometimes stumbles when letting out clutch from a dead stop.
6) Pinging when cruising on highway and air conditioner is on and ascending a slight grade.
7) gas mileage has dropped from 40mpg to 33 mpg.

Any suggestions on diagnostic tests that I can try to eliminate possibilities. Any help is appreciated.

BTW - +92000 miles on car. Only maintenance to date has been the required fluid changes (oil, coolant, etc.)

I am FAR from an expert....so I am passing on what I have learned from working on my 10 year old windstar.....and what I have read, while researching various problems.

A P0171 code is NOT caused by a downstream oxygen sensor.
It is possible, but not likely relative to an upstream sensor.

The downstream sensor will switch at a much lower speed than an upstream sensor. What it will read, I do not know.....but if there is any problem with the downsteam sensor, you will get one of a number of codes that specifically deal with the downstream sensor.

The Downstream sensor has ONLY 1 function.... Monitor the Catalytic converter, to make sure it is working. Also, can be called "Catylist Efficiency".

The upstream oxygen sensor is what your engine electronics (including the computer) uses to control the fuel mixture, etc.

A P0171 is a "lean code" and is most often.....far more than anything else......caused by a vaccum leak. Too much air....that the computer does not know about entering the engine combustion chamber.
I do not know the exacts of the toyota engine.....but the windstar has a Mass Air Flow (MAF) sensor. This is just after the air filter. This measures the airflow into the engine.

ANY air that enters the intake AFTER the MAF is air that the computer does not know about.
The PCV valve pulls air out of the crankcase......the air that flows INTO the crankcase to replace that air comes from the air intake.....after the MAF....so even that air has passed through the MAF, and is measured.
A vaccum line that has been bumped off, or has cracked can cause the P0171 code. Vaccum leaks can be difficult to track down.

Another, less likely, but possible cause is a dirty MAF.
To clean, disconnect, and use a non-residue electronic cleaner.
Again, I am not familiar with the Toyota engine.....so I cannot give you details on the MAF.

In short.....P0171 on ANY new car sold in the USA from 1996 to present means the same thing.....as that is a requirement of the OBDII standard......which is a requirement of all cars sold in the USA.

In short.....P0171 on ANY new car sold in the USA from 1996 to present means the same thing.....as that is a requirement of the OBDII standard......which is a requirement of all cars sold in the USA.

And in Canada as well.........................eh.

On a V type engine there is a sister code (174) for the other bank. If you have 2 banks and only have code 171 then the leak is currently only affecting one side of the engine at the present time.

The REAR HEGOs will switch MUCH slower (if at all) relative to the front HEGOs. The PCM will deliberately command a RICH or LEAN AFR temporarily and look at the rear HEGOs to see if they do switch. This verifies that they are working, and confirms that the PCM can count on them for accurate data.

Wiswind, DRW1000, ModMech - I appreciate all of ya'lls advice. Ya'lls input on the unmetered air and further research in other forums led me to pull the MAF and check it last night. It wasn't absolutely horrible but wasn't clean either. I brought home some Chemtronics electronics wash from work and gave it a good cleaning (on the Toyota, the MAF and IAT are together as one assembly so I had to make sure I cleaned both). In fact, I had to get fairly close and give it a good blast from the spray can to get some of the crud off of one of the two wires in the MAF - it (the crud) was pretty persistent in hanging on.

So, far on the drive to work this morning, things seem to be improved. The stumbling from a dead stop seemed nonexistent and there was no hard start. I am hoping that a couple of drive cycles will give the ECM a chance to get all of the fuel trims readjusted with, hopefully, a better MAF signal.

I'll monitor my gas mileage over the next week and post again. I'll also double check the possibility of vacuum leaks.

BTW - I had to change the Vehicle Speed Sensor on my '96 Winny over the last weekend and I took some photos. I will try to post them somewhere although I will admit that this will be a learning experience for me as I have never done it before. I doubt that the quality will be as good as Wiswind's but I hope that they may help someone.