Disconnect the wires from the thermostat to be tested. You will find two types of terminals used with thermostats, screw terminals or male-female push on terminals. In the case of screw terminals, loosen the screws with an appropriate screwdriver and remove the wires. In the case of male-female connectors, grasp the female push on-terminal with the needle nose pliers and pull them off. It does not matter which wire you connect to which terminal when replacing the wires on a two-wire thermostat, but if there are more than two wires, mark them with a small piece of masking tape on which you've written the terminal number, so you can reconnect them to the right terminals.
Set the "Function" switch on your DMM to the "Ohms" function. If your DMM is a manually ranging meter, set the "Range" switch to the lowest Ohms range. The lowest range on most manually ranging meters is a 200 Ohms. If your meter is an autoranging meter, it will select the appropriate range for you.
Clip one of the test leads from your DMM to each terminal on the thermostat. The meter will indicate an open circuit by displaying "OL" on its Liquid Crystal Display (LCD) for a good thermostat that has normally open (NO) contacts, or it will display zero (0.00) Ohms for a good thermostat that has normally closed (NC) contacts.
