Nissan Repair: still having problems..., fuel level sensor, vehicle speed sensor

Question
I have an 01 maxima that has a couple of problems...I got the codes read ad the idle sensor, MAF sensor, fuel level sensor and front left abs sensor came up..my main problem im having with the car is that the speedomoter does not work and it doeesnt keep track of the mileage either. When im idling the rpm constantly stuters a lot and sometimes drops and stays at 2 regardless if I hit the gas...eventually it'll start moving again though...my mechanic told me the idle sensor will fix that problem so  I changed it and im still having the same prroblem..also it seems like my car is drinking gas literally...u think any of those coded have an effect on my MPG?..also I forgot to mention that my car has an 02 transmission intead of an 01...please help me out as much as possible..im a colleege student and it kills me to keep having problems with my only source of transportation...thanks a lot...

Answer
Kalo,

First, replace the idle air control valve (but you already did that).  Second, the mass airflow sensor (MAF) needs to be replaced as that will really screw with the mileage.  Third, replace the vehicle speed sensor, that is located on the transmission (should not matter that it is from a different year).  Also, it could be that the VSS is not plugged in since the change or that it popped off.  You can see it from under the hood on the driver's side, looking down at the transmission.  The VSS and MAF will kill your mileage so get those replaced and reset the codes by disconencting the battery for five minutes.  A wrecking yard is a great source for parts since you are a student and cost is a factor.  Otherwise, rockauto.com or courtesyparts.com are both cheap places for new parts.