I replaced a White-Rodgers thermostat with a Simple Comfort 2000. The old thermostat had the yellow wire attached to the unit. No yellow wire is coming out of the wall. The new unit does not have a yellow wire attached but does have a terminal for it. Can I run it without the yellow wire or should I do something to make up for the fact that it is missing from the wall? Thank you for your help with this.

check the furnace end,,,,,the Tstat wires hook up to control board and are labeled as to what they are ,,,,,,,,,G=fan,,,W=heat,,,R=hot,wire with 24 volts that send power to the TStat,,,and yellow =a/c,,,this is the usual rule of thumb,,,,so,hook the tstat wire into tstat the same way,,,,,if you don`t have central air,,,you don`t need the yellow

The yellow wire usually runs to the coil of the A/C compressor outside. Do you have central air?

Thank you for helping with this, yes, I do have central air and I connected the thermostat as follows Green wire to G, White wire to W, Red wire to Rc with a jump to Rh and then connected the blue wire to Y. Initially, it didn't come on but I called a friend and he said wait because of a delay. It is working great and I didn't short anything out (which is great because my husband was already upset by my initial tampering). Thanks again for your help.

Check the sticky at http://www.askmehelpdesk.com/heating-air-conditioning/heating-cooling-maintenance-repair-troubleshooting-frequently-asked-questions-58313.html
I think it mentions sometimes it is a blue wire that runs from the Y terminal out to the A/C unit. You may be good to go. Just make sure the blue wire doesn't connect to com at the furnace.