How to Test a Four-Terminal Thermostat

Thermostats are used to control a number of household appliances and electrical systems. A four-terminal thermostat is most often used to control your home's central air conditioning and heating system. If your air conditioning or heating system is not functioning properly, it may be wise to check the thermostat before you call in a repair person to check the system itself. Sometimes, simply replacing a malfunctioning thermostat can solve the problem and save you money spent on unnecessary repairs.

Screwdriver
Paper
Pencil
Tape

Instructions

- 1
  Turn off the power to the air conditioning and heating system. Although a thermostat carries very low voltage, it is always a good idea to turn off the power to any electrical device before testing or repairing it.
- 2
  Remove the cover to the thermostat. The cover is typically held on by two to four screws that can simply be unscrewed to loosen or remove the cover.
- 3
  Make a diagram of the inside of the thermostat before you remove any wires to test them. Be sure to note which colored wire is attached to each terminal. In addition, be sure to use tape to secure any wires that you pull out or disconnect during the test so that they do not fall back into the wall.
- 4
  Locate the red and white wires. Red and white are typically used for heat, while red and green are used for cooling. If you are uncertain about which colors are for which system, consult your manufacturer's manual.
- 5
  Twist the red and white wires together. Turn on the power to the furnace. If the furnace turns on, then the thermostat is likely defective. Turn the power back off and conduct the same test with the red and green wires for the air conditioning if it is your air conditioning that has not been working properly.

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