The control will be connected by two wires. Label the wire placement on the control before disconnecting the wires. The wires are connected to the switch with slip on connectors. Firmly pull the connector (do not pull on the wire) off of the control's terminal. You may need to use a pair of needle-nose pliers to remove the connectors. Inspect the connectors and the terminals for corrosion. If the connectors are corroded they should be replaced.
Test the control for continuity using a multitester. Set the multitester to the ohms setting X1. Place a probe on each terminal. Turn the control to warmest setting or off. The multitester should display a reading of infinity. If the control does not have an off position, the reading should be near the infinity end of the scale. Now change the control to the coldest setting and retest. The display should read zero ohms. If the control does not pass both of these tests, the switch should be replaced.
If there is a separate control for the freezer and refrigerator, test the other control in the same way.