OK so I did A LOT of digging and here is what I have come up with and am trying. So on many models of the Ford Taurus
the Sun Load Sensor or Climate Control Sun Sensor is responsible for both interior temperature readings AND automatic light control. Additionally, on other vehicles, according to a mechanic from a local Ford dealership, the Ambient Temperature Sensor handles both internal and external temperatures. In the end the EATC computer does the math to figure out what temperature air needs to come out of the vents so that when it mixes with fresh air from outside will make the cabin temperature the temp designated. After that it monitors internal temperatures to figure out what compensation is needed due to body heat, solar radiation etc.
However, on the 2001 Ford Taurus SEL there are three sensors playing together in EATC. My ambient temperature sensor, which is responsible ONLY for external temperature readings my SunLoad sensor or Climate Control Sun Sensor, which is only responsible for automatic lights and my temperature control sensor which monitors cabin temperature.
The EATC code 030 indicates my temperature control sensor has a constant short which is why I only get max heat or max A/C. Code 051 indicates that my sunload sensor's circuit is open and thusly needs to be replaced.
My Ford Dealership was going to charge me $230 + the part to change the ambient temperature sensor -- rip off. The sensor itself is $20 they wanted $54 and it would have been wrong.
Finally, the how to. The sun load sensor is located in the defroster on the dash -- there are plenty of tutorials online that show you how to unscrew the two mounting screws, unplug the sensor, and hook up the new one. To remove the defroster shield simply use a plastic puddy knife.
The temperature control sensor is located in the EATC head right to the left climate controls. There is a "vent" right next to the display that allows air to be drawn in -- the sensor is behind the vent. Go to your local autozone and pick up OEM stereo removal clips. Push them in to the four holes on the EATC head to remove the unit -- underneath the vent you'll see some wires and a hose going into the sensor. Pull the sensor out, hook up the new one and re run EATC diagnostics. Should reset everything and your good.
Anyway this is all theory -- I won't know until I get the sensors in and can try it out, but it's definitely A LOT further than I was last night.
Thanks for all of your help.