If you do not have experience with HVAC systems you would be better off getting a experienced HVAC technician to check your thermostat. You can cause additional issues to your thermostat and or HVAC system that will cost you more money than hiring a HVAC technician. You can also cause harm to yourself by working on HVAC systems if you do not know what you doing.

How to check a thermostat.
First turn the fan switch on at the thermostat. This is a quick check to find out if the thermostat has power and if the fan will come.

If the fan does not come on check to make sure the breakers are on for the indoor air handling unit and the outside condensing unit. Remove the thermostat cover to get to the thermostat sub base. The sub base is the part of the thermostat that has the terminals where the thermostat wires are connected to.

If you have a multi meter set the meter to volts A/C and check for 24vac between the R terminal and the C terminal on thermostat sub base. If you do not have a multi meter place a jumper (a jumper can be any wire gauge size 20 AWG and bigger that has a sheathing around the wire) on the R terminal and G terminal to see if the fan will come on. Make sure not to let the jumper wire touch any of the other terminal especially the C common terminal. You will burn up the transformer by touching the jumper wire connected to the R terminal to the C terminal.

If there is no power on the R terminal when you check it with your meter or if the fan dose not come on when you jumper the R terminal to the G terminal you need to make sure that the indoor air handler has power. There may be a switch by the air handler that may be off. Some air handling units have a switch on the air handler door that cut off the power to the thermostat if the door is not closed. The transformer in the air handing unit that sends power to the thermostat may be bad. After all of these items have been checked it would be best to call a HVAC technician.

If you jumper the R terminal to the G terminal on the thermostat sub base and the fan come on but will not come on when you turn the thermostat fan switch on your thermostat need to be replaced.

If you jumper the R terminal to the Y terminal on your thermostat sub base and the outside condensing unit comes on but the outside condensing unit will not come on when you set your thermostat to a much lower temperature than the house temperature you need to replace your thermostat. Check you thermostat wire

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