What sensors are available to be read by and OBDII scanner?

[thebasemetn1]

I recently purchased a 2007 Yaris S sedan with a ton of options and a bluetooth OBDII Scanner. With the OBD I don't seem to be picking up a MAF reading, there is only 1 oxygen sensor being displayed (there should be at least 2), the vehicle speed (mph) is sometimes negative (as much as -37), and the intake air flow rate (lb/min) is always 0. I'm not getting any trouble codes, but that just seems weird to me. I was hoping to track mpg using the OBD, but I need MAF readings to do that. I know the scanner works, because it worked on another car I had and everything made sense, except for the intake air flow rate, which was again always 0. I did call a Toyota dealer about the MAF and they said the 2007 Yaris doesn't have one. That doesn't seem right. So, does anyone know why the above would happen and is there a list of sensors one should be able to access with an OBD?

[Stevo_Echohb05]

Your Yaris should have an intake air flow meter located on your airbox.
as for calculating MPG take your total Miles and divide by total Gal

[thebasemetn1]

Thanks for the reply Stevo_Echohb05. I should have been more explicit. I was looking at a Mass Air Flow (MAF) sensor because I am interested in INSTANTANEOUS mpg readings (like one can get with a scanguageII), so that I may better train my driving habits to reduce fuel use. I am aware that if I divide the miles I drive by the amount of gas I use I get mpg . Do you know where the air intake box is or what part of the system it is in? I can't seem to find a reference to its location, nor am I familiar with it.

[Stevo_Echohb05]

Open the hood, you should see the air air box connected to the motor via a large black hose, the MAF is screwed onto the top of the air box.