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.
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.
Check the circuit breaker panel.
The next simplest solution for a thermostat that has no power is a tripped circuit breaker. This is a common cause for appliances, power outlets, and HVAC equipment not working. Open your electrical panel and look for a switch that is not aligned with the rest.
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.
Pull the thermostat off the wall and jumper the red and green wire. If it turns on, thermostat is bad. If it doesn't, something else is going on.
If you get a “The C wire isn't getting power” error code during power test, turn off your HVAC power at the circuit breaker. Remove the thermostat display and make sure that the C wire is fully inserted then turn the power back on. If the C wire still isn't getting power, we recommend contacting a Pro.
Yes, the thermostat stores the set point and schedule. When power is lost to the thermostat, the only information it "loses" is the current time. Once power is restored, the thermostat will resume its previous settings and configuration.
For those people who might not be familiar with wire gauging, low voltage thermostat wires are more similar in size to angel hair pasta, and high voltage wires are thicker like linguini. Aside from size, line voltage thermostats often – but not always – have four wires.
Faulty Wiring or Equipment: Bad wiring can prevent power from reaching your thermostat. It can also increase the risk of a short circuit or fire. Check whether the thermostat's wiring is frayed, broken, or installed incorrectly (the wires/terminals are usually color-coded).
One of the most common reasons why a thermostat may fail to turn on the air conditioning system is dead batteries. If the batteries in your thermostat have died, it may not have enough power to communicate with the air conditioning system and turn it on.
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.
Use the Right Tester: If you're checking an electrical outlet or a socket, a non-contact voltage tester is the safest way to determine whether a wire is live. For more detailed readings, consider using a digital multimeter.
Use a voltmeter to measure 24 VAC between the C and R wires. If you don't get anything then either the fuse inside the 24 volt control transformer is blown or there is no power to the AC. If you can see 24 VAC on those two wires, then the C wire is good.
If your thermostat doesn't detect a wire, or has a power issue, you should check that the wire end is straight, fully inserted into the wire connector, and isn't corroded.
If you try to adjust the temperature settings and nothing happens, or there's a significant delay, your thermostat may be on the brink of failure. Possible Causes: Electrical or wiring problems. Software issues in digital or smart thermostats.
If your thermostat is battery-powered, the first step is to check if the batteries have run out. Replace them with fresh ones and observe if the screen comes back to life. In the case of a wired thermostat, a tripped circuit breaker or a blown fuse in your home's electrical system could be the cause.
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.
Replacing a Thermostat. You'll pay between $15 and $300 for the unit in addition to labor ($65 to $100 per hour) to replace an old thermostat with a new one.