[wiswind]

Speed sensor, there are 2 of those.
1, the most common to be discussed here (also the most difficult to change) is the Vehicle Speed Sensor (VSS) which is what the speedometer connects to.....and also has an electrical component that tells the PCM how fast the vehicle is moving.
The other speed sensor is the Turbine Speed Sensor (TSS) which tells the PCM how fast the shaft coming out of the Torque Converter is spinning.

There may be additional codes that they cannot read at autozone.
The Generic OBDII code readers read the "P" codes, which are the same over all vehicles manufactured and sold in the USA starting in 1996 and newer.

There are still Proprietary codes (including "C") codes that require a reader that can read manufacture's specific codes.
Your local dealership will have such equipment.
They will charge for this service, but it is well worth the cost to properly diagnose what is going on.

Many other repair facilities will have this equipment also, but that is hit and miss, depending upon how much call that they would have for that particular item of test equipment.

Often times this equipment costs thousands of dollars, which makes it impractical for the average DIY person to purchase to diagnose 1 vehicle.......and most likely it would not work on the next vehcile that you buy.