A thermostat is a current regulator that is typically used in heating and cooling appliances. It is comprised of a bimetallic coil that expands and contracts under the influence of temperature differences connected to the opposing ends of a mercury-filled bulb. The sensing of temperature changes occurs when the two metals expand or contract at their individual expansion rates. This causes the two metals to pull or push the bulb until mercury flows into the empty end and makes contact with the opposite terminal, completing the circuit. An ohmmeter may be used to check the circuit for continuity when a faulty thermostat is suspected.
Unplug the appliance to reduce the risk of electrical shock. This is a first step to all appliance repairs unless live current monitoring is necessary.
Disassemble the appliance, using the screwdriver to remove the outside panel(s) and any components that obstruct access to the thermostat. If the appliance contains a capacitor in its circuitry, manually discharge it by placing the metal end of the screwdriver between the ground terminal and the capacitor's diode connection. Do this only if the handle of the screwdriver is insulated.
Disconnect the wiring on the thermostat. If the thermostat is one that is easily replaced, you should be able to un-clip or pull the contacts out of the terminals so that reconnecting the circuit doesn't involve soldering the connection back together.
Turn on your ohmmeter and set the dial to the continuity setting. Touch the probes together to ensure that the ohmmeter beeps on a complete circuit.
Touch the ends of the probes to the terminals of the thermostat. A working component will cause the ohmmeter to beep. If the circuit is incomplete (the ohmmeter doesn't make any sound), the thermostat is faulty and must be replaced.