Turn off the furnace's circuit breaker. Untwist the “R” and “W” wires, remove them from their connectors, and then twist the wire ends together. Turn on the furnace's breaker. If the light comes on, it shows that the thermostat is not sending a signal to the furnace.
To test them, switch your thermostat “off,” and then switch back to “on.” Set it to the heating or cooling mode, and turn it up or down, depending on the season. Some thermostats will make a clicking sound when they turn on — you might even hear your HVAC system click on as well.
If there is a problem with the high voltage wire, the thermostat will not be able to signal the HVAC system properly. A high-voltage wire that is damaged, loose, or disconnected causes thermostat problems.
You can determine if the wiring is defective by turning the fan from the “Auto” setting to the “On” setting. The blower should start, which means your thermostat is getting power. Or, if the AC is on, turn the temperature down; if the furnace is on, turn the temperature up.
If the thermostat runs from the house's electrical system, check on the circuit breakers. One of them might have tripped and cut off the power the thermostat. Reset the breaker. If this doesn't work, then the issue with the thermostat could come from loose connections or other wiring problems.
If your thermostat is showing a 'No Signal' message, it means that it has lost connection to the receiver.
Standard thermostats: Enlist a friend or family member to assist, Position one person by the thermostat and one person by the furnace. Slowly turn the thermostat from off to heat and increase the temperature. Both the thermostat and furnace should make a sound.
The common wire, sometimes referred to as the c-wire, is what's responsible for delivering constant power to your thermostat. For programmable and smart thermostats, c-wires are required for setting a heating or cooling schedule.
Check the Power Source Sometimes the problem between the thermostat and your furnace is as simple as replacing the batteries. Many digital thermostats use standard batteries to power their functions. Also make sure to check there is power at the furnace as well.
There is usually a small reset button somewhere on the thermostat. Finding this button may require that the thermostat panel be removed and pressing this button may require something like a paperclip to push. Hold the reset button down for 5 seconds and the thermostat should reset.
A bad thermostat can be detected when your AC or furnace is not turning on, your HVAC systems are not turning off, the thermostat not responding, your HVAC system is short-cycling, the room temperature not matching the temperature displayed on the thermostat, and the programmed setting resetting themselves.
Replace Dead Batteries
While some thermostats are hardwired, many use batteries or have batteries as a backup power source. Dead batteries are one of the most common reasons a thermostat may fail to turn on the air conditioning system, leading to a loss of connection and inability to activate the HVAC system.
Residential HVAC systems are typically equipped with a single thermostat. In most cases, these are located on an interior wall, about midway between the floor and the ceiling. This provides the best location to measure and control to an average temperature.
Place the multimeter's red probe onto the thermostat terminal marked R for red; you can place the black probe onto any of the other terminals. The multimeter reading should be at or close to the thermostat's voltage level.
Typically, a home thermostat utilizes five wires that connect to HVAC components and power. These wires are color-coded for easy identification. The red wire is the “hot” wire that carries 24 volts. The white wire carries 24 volts to the gas controls in the furnace when the thermostat signals for heating.
Safety Switch Has Been Tripped
Sometimes, a problem with the safety switch on your AC can cause the thermostat to lose power. Most AC safety switches are next to the drain pan. Excess moisture can trip this switch, causing the transformer to stop supplying power to the thermostat.
The power must remain off for this test. To test for an internal short in the control board, remove the fuse and the thermostat wiring from the thermostat terminals on the control board. At this point, take a resistance reading between the C (common) terminal and the R (power supply) terminal on the board.
If you have two wires, R or RH go into the R terminal and RC into the RC terminal. If you have more than one wire (for example, you have a wire labeled R and another wire labeled Rc), remove any jumpers between the R and Rc terminals or push the switch to open the RC terminal to insert a wire.
A "No Signal", "No Source", or "No Input" message will appear on your TV screen if the TV is not receiving a signal from your TV box. This is often a result of either the TV box being powered off, not being properly connected to the TV, or the TV being set to the wrong input.
A quick fix to restore your thermostat's power is a tripped circuit breaker or blown fuse, depending what you have in your home. Sometimes a surge can cause this issue. If you check your electrical box and a circuit breaker is tripped, just flip it back. If you have fuses, replace the fuse.