Megan's 4 runner threw a code a little over a week ago. I took a look at it last weekend but the water heater took precedence. The code was a P0171 which reads "system too lean (bank 1)". I googled it and it looks like its either a mass air flow sensor, a vacuum leak or an O2 sensor.

Anyone have any good ideas on how to narrow down the search? I tried spraying carb cleaner on the MAF sensor last weekend but it came back on.

To top it off, the registration expires the 28th!

Funny thing,  I got home last night and she had gone to autozone to have the code checked and see if they could shed light on it. They checked it, said the same things we knew and tried to sell us carb cleaner and seafoam. They also cleared the code. I hooked my scanner back up cause I found where it does live data. I went through it and the MAF sensor was showing varying numbers, the O2 sensors were reading a few tenths of a volt. Everything looked good. I will have to wait until it throws a code again and then check those.

The STFT had NA next to it. I'm not sure what that means, but it was the only thing that looked to be different

only one of them! (see above)

My Volvo engine light comes on about 2 weeks after clearing codes. Nobody seems to know whats doing it.  Here is what I do. Get the codes cleared and right then and there go  though emission check before the light comes back on. I have done that for 2 years

Just because the O2 sensor is reading 0 volts doesn't mean it's dead.  A typical O2 sensor generates a small amount of voltage based on the differing amounts of oxygen in the exhaust and the outside air.  When the difference is large between the two, engine running rich, it can generate right around 1 volt.  When the difference is small, engine running lean, it won't generate voltage.

Normally, when you look at the voltage output with a scan tool or a voltmeter the readings will fluctuate between 0 and 1 volt rapidly.  If you look at it with an oscilliscope it should look like a sine wave.  So the engine is constantly switching between running lean and running rich.

The reasoning behind this is to keep the catalytic converter alive.  Think of your converter as an oxygen storage device.  When the engine goes to the lean side of things the converter stores the excess oxygen that's in the exhaust stream and when it goes rich the converter releases the oxygen to burn the excess fuel.  If it didn't do this the converter would never reach operating temperature.

There are several things that can cause the O2 sensor not to generate any voltage.  Some of the usual ones are; vacuum leaks, exhaust leaks, low fuel pressure and, of course, a dead O2 sensor.  Check all your vacuum lines and make sure they're ok.  Make sure you don't have any exhaust leaks.  Fuel pressure is pretty low on the list since your truck would probably be running poorly if it wasn't getting enough fuel.  Finally, if the O2 sensor has never been replaced, it's time to replace it.  I'd advise against cheap Auto Zone/Advance/Pep Boys parts.  They're cheap for a reason.  If the original one lasted this long it can't hurt to replace it with another one just like it.