If the your heating, ventilating and air-conditioning unit is not coming on or shutting off at desired temperature settings, you could have a faulty thermostat. The convenient way to know whether your thermostat is at fault is to test it with a volt-ohm meter. If you do not have a volt-ohm meter, you need to know how to bypass a thermostat to test it.
Turn the circuit breaker off to the HVAC system.
Use a flathead screwdriver to remove the thermostat cover plate. There will be a slot to place the blade of the screwdriver between the faceplate and the main body of the thermostat. Insert the screwdriver in the slot and twist slightly to pop the faceplate off.
Use a Phillips-head screwdriver to remove the mounting screws that secure the thermostat to the wall. Use small pieces of masking tape to label the wires to the thermostat. The terminals on the thermostat are stamped with letters R, W, Y, G and C.
Use a screwdriver to unscrew the thermostat terminals and remove the wires from the thermostat. Do not allow the wires to fall inside the hole in the wall.
Twist the R wire and the W wire together and make sure that the other three wires are not touching one another. Turn the circuit breaker to the HVAC unit on. The furnace should start when the power is connected. Turn off the circuit breaker and untwist the R and W wires.
Twist the R wire with the Y wire and make sure that the other wires are separated. Turn the power back on to the HVAC and check to see whether the air conditioning system comes on. Turn the power off to HVAC after the test and untwist the wires.
Twist the R wire and the G wire together. Turn the power on to the HVAC at the circuit breaker. The fan should now be operating. Turn the power supply off after the test and untwist the wires. If the HVAC passed all of these tests, the thermostat is faulty and needs to be replaced.