Air control knob doesn't work on 1, 2, or 3.

My air control knob (not hot/cold control) has quit working on 1, 2 and 3 but 4 works fine. What are some possible reasons why this is happening? I have a 2008 Ford Escape XLT V6.

Go to any parts store and ask for a new heater fan resistor block. They probably have one in stock. Average around $20. Now the fun part, where is it? On my older it is mounted through the heater box with two screws and the wire connector on the outside. Under the dash, passenger side to the right of the glove box. On other cars I've seen it to the left of the glove box. Having the new one helps to have an idea of what to look for.