Troubleshoot Nest thermostat power errors when it's hot outside

During the hot summer months, if your heating and cooling (HVAC) system breaks down it may stop sending power to your Google Nest Thermostat. When this happens, your thermostat will display an E3, E4, E23, E24, E73, E74, or E195 power error. If this problem persists, your thermostat’s battery will drain and your thermostat might disconnect from the internet or shut down.

Important: If you see one of these errors in the summer you’ll likely need to contact an HVAC professional.

If your Nest thermostat worked without any issue before the hot summer months here are a few things to consider:

  • Check that your breakers are turned on. If you’ve had any construction or maintenance work done on your home, your breakers might still be off.
  • AC units fail more often during the hot summer months. If your AC fails, you'll likely see an error on your thermostat. It’s a good idea to have an HVAC professional come take a look at your system and have them perform regular maintenance.

What to do if your thermostat screen is black and won't turn on 

This can happen if your thermostat has been without power for long enough that the battery is dead or if you’ve just installed your thermostat and there is no power to your thermostat wires so it can turn on.

Remove your thermostat from the base by gently gripping the thermostat ring and pulling toward you, away from the wall. The thermostat display should pop right off the base. Charge your thermostat with a USB cable. Newer Nest thermostat’s use micro USB, the same type of charger as many cell phones, cameras, and other electronics.

Charge your thermostat for at least 10 minutes, and reattach the display to the base. This will give your thermostat enough power to turn on and give you an error message.

Even if you can’t get your thermostat to turn on, it’s a good idea to continue with the troubleshooting below.

 Troubleshooting

1. Check your thermostat's wiring 

Important: Turn off the power to your HVAC system at the breaker and remove your thermostat display. Your HVAC system can have multiple breakers so make sure to turn them all off before moving on.

Inspect each of the wires installed on your Nest base.

thermostat backplate wire example

  1. Remove one wire and inspect it.
  2. Make sure it's straight with at least 1 cm of copper exposed.
  3. Make sure the wire is clean and free from paint or corrosion.
  4. Insert the wire fully back into the connector of the Nest and make sure the connector button stays pressed down.
  5. Repeat with all of your wires.

Turn the power back on and test your cooling system. If you still have trouble, read below to check your drip pan and HVAC fuse.

2. Check your filter 

It’s important for your system to have good airflow to run properly. A clogged air filter in the summer can limit how much air flows over your cooling coils. If this persists your cooling coils can freeze solid and cause your system to shut off.

Find your system’s air filter. It’s usually behind a grate in the hallway. Check along your walls and ceiling. Your air filter may also be inside your furnace near the fan. If your filter looks dirty or clogged, replace it with a new one. If your issue was caused by frozen cooling coils, you may need to wait a while for your system to thaw before you can use it again.

If you need more help finding your air filter, check your systems user guide or contact a professional.

Important: If your air filter is in your furnace, make sure to turn the power off at the breaker before changing your filter.

Important: The following steps are advanced troubleshooting steps. Your heating and cooling system uses high voltage  which can be dangerous. Make sure to turn off the power to your heating and cooling system completely. Contact a local pro if you need any help.

3. Check your drip pan or drain tubes 

Air conditioners (AC) and heat pumps work to cool the air in your home. Like a glass of ice water, water droplets condense on the cold coils of your AC or heat pump and drip off. Usually this water drips off the coils into a drip pan and flows down a tube outside your home or down a drain.

If your drip pan or drain tube becomes clogged and water starts to back up, your AC or heat pump may shut itself off to prevent an overflow of water. When this happens your system stops sending power to your Nest Thermostat and causes an E3, E4, E23, E24, E73, E74, or E195 power error.

  1. Turn off the main power to your HVAC system at your breaker box. Your HVAC system can have multiple breakers so make sure to turn them all off before moving on. Once the power to your whole HVAC is off, check your furnace and fan. Make sure there are no lights that might indicate your system still has power.
    • If you still see lights on your furnace or fan, your system might still have power. Turn off the main power to your home at the breaker.
  2. Look for your cooling coils to help find your drip pan. Cooling coils sometimes look like a pyramid on top of your furnace. You can consult your manufacturer's manual or search online for more information on finding your drip pan and drain tubes.
    • If your cooling coils are behind a panel sealed with metal tape or sealant, you should not remove this panel yourself since your system may not work as well without the seal.
  3. Usually underneath your cooling coils you can find your drip pan. Connected to your drip pan are one or more drain tubes. Check to see if there’s any water in your drip pan or if your drain tube is clogged.
    • Some drip pans can be to the side of your furnace or on the ground.
  4. Look for your drain tubes. These are usually plastic tubes that carry water from your AC unit to a drain or to outside your home. These tubes are usually on the side on your furnace.
    • Some AC systems have a safety valve installed on the drain tube, sometimes called a float switch or drain switch. This is usually attached to your furnace by a thin wire. Your float switch can usually be removed to check if there’s water in the drain tube.
  5. If you find water in your drip pan or in your drain tube, you may have a clog. Also look for any signs of water damage or rust that might show where water has been before. Consult the user guide for your HVAC system or search online to learn how to clear a clogged drip line or contact a local pro in your area.

4. Check the HVAC fuse 

If your AC or heat pump has had to work extra hard because it’s hot outside, because your filters are clogged, or because of age, your fuse is more likely to burn out. If your fuse burns out your system may stop sending power to your Nest Thermostat and cause an E3, E4, E23, E24, E73, or E74 power error.

1. Turn off the main power to your HVAC system at your breaker box. Your HVAC system can have multiple breakers so make sure to turn them all off before moving on. Once the power to your whole HVAC is off, check your furnace and fan. Make sure there are no lights that might indicate your system still has power.

  • If you still see lights on your furnace or fan, your system might still have power. Turn off the main power to your home at the breaker.

2. Look for your HVAC fuse. The fuse for your HVAC
system is usually on your furnace control board. If you
need help finding your control board, consult your owner’s
manual.

  • Fuses can be different for each type of system.
    You may see a small U shaped fuse that is labeled
    3 and is pink or purple colored. You might also see a round fuse.
fuses for HVAC systems

3. Remove and inspect your fuse for damage. If your fuse looks blackened or discolored, consider replacing your fuse.

  • To learn more about replacing your fuse, consult the user guide for your HVAC system or contact a local pro.
  • Make sure to reattach any panels you needed to remove on your furnace as your furnace may not turn on if your panels are not attached correctly.

Other problems with your AC or heat pump

If you’ve removed any panels on your AC or furnace, be sure to put them back. Remember to turn the power back on at your breaker. Your furnace won’t work if these panels are missing or not firmly in place.

If you checked your drip pan, the fuse, the air filter, and the wiring to your thermostat but there’s no problem, you should contact a local pro. They can help figure out what’s wrong.

 

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