No Power to Thermostat? 9 common causes and their fixes.

A thermostat is a crucial part of your HVAC system as it literally tells the unit what to do and when to do it. Unfortunately, there might be times when this important device stops receiving the power that it needs.

No Power To Thermostat? First, make sure that the device is actually on. After that, check the furnace circuit breaker(s) and, if tripped, reset them. Next, replace the thermostat’s old batteries, clean the device, and check the wires and connections.

Where Does a Thermostat Get Its Power?

The old models didn’t need their own power supply. However, the newer thermostats require constant power that, in general, comes through the thermostat C wire (common wire).

No power to thermostat - older model

This extra wire creates a continuous 24 V power loop between the smart thermostat and the rest of the system.

Fact: the C wire can be of any color and is not always labeled.

However, the common wire is not the thing that powers the thermostat. In the majority of cases, the hot wires labeled Rc and Rh provide 24 V to the device (they also ‘travel’ to the HVAC system’s control board).

Below is a chart of the most common thermostat wiring color configurations. But DO NOT ASSUME YOURS IS THE SAME. It is always possible someone used a non-standard coloring method.

thermostat wiring color code

See Systoncable.com for more info on thermostat wiring terminations.

Read: Why Does My Thermostat Say ‘HEAT ON’ But There Is No Heat?

Is There a Fuse for the Thermostat?

A fuse is a safety device that helps make sure that the circuitry is not drawing too much current.

Thermostats do have a fuse. To locate it, open the device’s panel and look for a small, clear cylinder with a filament running right through the center.

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Thermostat fuse location

Just like any other fuse, your thermostat’s fuse can get blown. This will cause the device to malfunction. Be sure to replace a bad fuse with one of equal value.

Do All Thermostats Require Power?

If your thermostat has a colorful LED screen and/or a Wi-Fi function, then the chances are high that it is going to need power in order to function.

However, there are extremely simple models (usually, the older ones) that can run on batteries only.

Thermostat with digital display

Some modern battery-operated thermostats still need a C wire that is going to supplement the power in the event that the battery runs out.

Hint: Typically, a battery-operated thermostat requires AA or AAA alkaline batteries, a 9-volt battery, or a 3 V lithium battery.

Read: What Type Of HVAC System Do I Have?

Why Did My Thermostat Suddenly Stop Working?

  • Wrong settings

First things first: If you feel like the thermostat has stopped working, you should check its settings. You might have accidentally switched the device to ‘cooling’ or ‘heating’ mode.

Also, make sure that the temperature settings are appropriate. Bear in mind that if you have drastically lowered or increased the setting, it might take your HVAC system quite a while to reach the desired temperature.

Read: Why Does The Room Temperature Not Match The Thermostat Setting?

  • Loose wire connections

Wires can become loose or broken/corroded over time. If the wires inside the thermostat or the ones running to the HVAC system look bad or disconnected, you would have to fix the issue (call a professional, if you don’t have experience dealing with electricity).

thermostat location is key
  • Wrong thermostat location

At times, the device might be working perfectly fine, it’s just that the thermostat’s placing is not right. For example, if the device receives a lot of direct sunlight, it might think that the temperature in the house is much higher than it actually is.

As a result, the thermostat will fail to send the right signals to the HVAC system.

  • Blown fuse

If you can see that the device is on, but nothing is happening, then a blown fuse might be to blame. 

Remove the cover and inspect the fuse. If the filament that is running through the fuse looks broken, then it would have to be replaced.  If you can’t tell visually, you can perform a continuity test with a multimeter or ohmmeter.

Why Did My Thermostat Go Blank?

  • Dead batteries

Replacing the old batteries with fresh ones is one of the easiest solutions. 

thermostat battery location
  • A tripped breaker

Head to your breaker panel and locate the circuit breaker for your furnace. In the event you spot a tripped breaker, simply flip it fully OFF and then back ON, and see if this solves the issue.

Bear in mind that serious problems within the HVAC system can cause the breaker to trip, so if that happens again, you might want to consider calling an expert.

  • A tripped safety switch

A malfunction in your HVAC system can make your thermostat go blank (the transformer will stop sending voltage to the device).

This might happen if the system’s safety switch trips. In an AC system, this part is located right next to the drain pan.

If this is the issue, then calling an HVAC expert might be the best decision. 

  • Low voltage

A bad transformer or faulty wires that run to or from the transformer can cause the thermostat to receive low voltage or no power at all. Corrosion and pests can lead to such issues.

thermostat wire through hole - no grommet

Make sure that the wires look correct. If there are places along the wire where the sheathing or insulation of the wiring is damaged, a short could exist. If so, call a professional to deal with the problem. 

  • A broken thermostat

Finally, there might be an issue with the thermostat simply because the device is too old or faulty.

The average lifespan of a thermostat is around 10 years. If you have had the device for quite a while already, then it might be time to replace it. 

How Do I Restore the Power to My Thermostat?

This easy step-by-step guide will help you restore the power to the device (or at least help find the root cause of the problem).

  1. Check if the thermostat is on (you might have bumped into the device and accidentally turned it off).
  2. Check the house’s power supply. Inspect the circuit breaker and flip the breaker back, if it had tripped.
  3. If your thermostat has batteries, replace them.
  4. Reset the thermostat. See instructions below*
  5. Make sure that the fuse inside the thermostat has not been blown.
  6. Clean the thermostat. Remove the cover and use compressed air and a soft brush to get rid of the dirt.
  7. Check if the device is wired correctly. Find the R, C, G, Y, and W terminals on the control board and make sure that they have the corresponding wires plugged into them.
  8. While you’re there, inspect the wiring. Ensure that the wiring is not corroded or damaged.
  9. Check the transformer and the wiring running to it.

If you have noticed that the wires are damaged, the transformer looks bad, or the fuse had been blown, then you might want to call a professional to deal with the problem.

*Before proceeding to steps 4-8 mentioned above, you might want to try and reset the device.

Simply hold down the reset button for at least 5 seconds. If you have a battery-operated thermostat, then you can flip the battery directions for around 5 seconds and then flip them back.

If that didn’t help, reset the circuit breaker and wait for about 30 minutes before turning the thermostat back on.

No Power to Thermostat – Carrier

Once you have checked the house’s power supply and replaced the batteries, you can try resetting the device.

To reset a Carrier Cor thermostat, you would have to tap ‘Menu’, then select ‘Settings’ and ‘Reset’. You will see a lot of reset options, pick ‘Reset All’ and ‘Yes’ to confirm.

No Power to Thermostat – Ecobee

If you have had your Ecobee for quite a while, then check the power supply.

However, if it’s a new thermostat, then there might be something wrong with the wiring. Check the device’s placement on the backplate – the wiring connections should be snug and correctly terminated.

There should be a C wire and the power R/Rc/Rh wire has to be connected to the Rc terminal. Ecobee3 is the only exception.

No Power to Thermostat – Honeywell

Follow our step-by-step guide that you’ll find above.

Also, make sure that the thermostat’s panel is pressed in perfectly. Otherwise, your Honeywell thermostat is likely not going to work.

Read: Honeywell Thermostat Not Reaching Set Temperature? Here Is Why

No Power to Thermostat Goodman Furnace

If none of the tips above helped and you want to be certain whether your Goodman thermostat is receiving any power, then grab a multimeter.

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Set the multimeter to the voltage option, turn the meter dial to 24 V(AC), and touch the R terminal with the meter probe (the other probe should be touching another terminal, typically C). The reading should be between 22 and 28 volts.

Read: Goodman Furnace Error Codes – Troubleshooting Guide