99 24valve idol speed control sensor ?

I have a 99 Tauras 24Valve and the check engine light has been on for some time now. Auto Tune hooked up their computer and said it was my idol speed control sensor. The car has 110K miles and runs fine (i think). Here are my questions.
1. What should my RPM's be at 70mph?
2. How much will this repair cost and should I have it done if the car runs ok?

I am not sure if my milage has gone down because I never really got great milage to begine with approx 22 on highway. will this affect my milage?

any help would be great.

Chances are, they meant the Idle Air Control motor, or IAC. They get carboned up and stick occasionally. I would suggest cleaning it with carb cleaner and reinstalling. If it's not operating properly, your idle should surge.

Is that the little square box mounted on top of the head?

Nope, that box is the IMRC (intake manifold runner control) box. It controls the opening and closing of the secondary intake butterflies on the car.

The IAC is located on the top of the upper intake. It's about 1.25 inches in diameter, and roughly 3" long. It bolts to the upper intake with two bolts, and has a wiring harness plugged into it. A lot of times, these get carboned up, and malfunction. Usually all that's needed is to remove it and clean it with some carb cleaner spray.

What was the code that the computer stored for the check engine light?