An E73 error means your Nest Thermostat isn’t detecting your Rc wire electrically which could be due to a number of reasons. This can be resolved by checking your HVAC wiring, AC breaker, air filter, drain tubes/drain pans and ensuring they’re all properly working. 

A summary table of error E73 and to how to fix it

What is the Rc wire and how does it work?

The Rc wire is found on the wall at the back of your Nest thermostat and is denoted by the letters Rc and simply means Red cooling since it’s mostly red in color. It’s a live electrical wire connecting to the RC terminal that controls the cooling system by supplying your thermostat with volt power from the HVAC.

So when your Rc fails to deliver power to your Nest thermostat, then it fails. This could be caused by a number of reasons depending on the season – winter or summer. The most common reason is because of our outdoor AC unit/compressor shutting down due to issues such as filters blocking, overheating, outdoor fan motor issues or low gas.

How to fix

Check your wiring

Steps

1. Turn off power to your HVAC system by turning off your breaker. Your HVAC may have several breakers, ensuring they’re all turned off.
2. Remove your thermostat display from base and disconnect the Rc while maintaining the rest of the wires. This will make it switch to heat mode only.
3. Remove your Rc wire and inspect it by making sure it has at least 1 cm of copper exposed and it’s straight.
4. Ensure the wire is not obstructed by corrosion or paint.
5. Use a voltmeter and test the Rc wire’s voltage to ensure it’s consistent. A 24vac voltage is an indication of your Nest thermostat receiving power and the problem lies with the AC unit since most units often fail during hot summer months.
6. Insert back the wire into the Nest connector and ensure the connector button stays pressed down.
7. Just to be on the safe side, you can perform these steps on the rest of the wires.
8. Turn on your breaker to put on the power.
9. Put your Nest thermostat back on the base and wait for it to power on.

Check the filter

Your system needs a good airflow to maintain its performance. A clogged air filter can limit the amount of air flowing through your cooling coils especially in the summer which can freeze and cause your system to shut down.

Steps

1. Locate the air filter on your system usually found behind a grate in your hallway and can also be found along your walls or ceiling. It may also be located inside your furnace near the fan.In case it’s located in the furnace, then ensure you turn off power at the breaker first. If you have trouble finding the air filter, try checking your system’s user guide or contact your professional.
2. If your filter appears clogged or dirty, then replace it with a new one.
3. If the problem is frozen cooling coils, then give your system some time to thaw before using it again.

Check the drain tubes/drip pans

When your AC and heat pumps work to cool your home’s air, water condenses and drips off into the drip pan then flows down a drain or tube to the outside. In the event your drain tube or drip pan clogs, the water will start to back up causing your AC or heat pump to shut down in a bid to prevent a water overflow.

Your HVAC system will stop sending power to your thermostat which brings about the E73 error. To resolve this, follow the following steps and troubleshoot.

Steps

1. Power off your HVAC system by turning off the breaker. If your system has multiple breakers, ensure they’re all turned off before proceeding. Any lights in your fan or furnace might be an indication that your system still has some power.
2. Locate the cooling coils to help you find the drip pan. Cooling coils may sometimes resemble a pyramid on top of your furnace. If you have trouble finding it, you can search for more information online or check with the manufacturer’s manual.If you locate your cooling coils behind a panel sealed with a sealant/metal tape, do not remove the panel yourself because the system may not work properly without the seal.
3. Usually the drip pan is found underneath the cooling coils connected by one or more drain tubes. Ensure your drain tube is not clogged and there’s no water in the drip pan.

In some, the drip pan may be on the ground or the side of the furnace. Find the drain tubes which are usually plastic tubes located on your furnace’s side carrying water from your AC unit to the outside or to a drain.

Some ACs contain a safety valve sometimes called a drain/float switch in the drain tube attached to the furnace by a thin wire. Remove the switch and check if there’s any water in the drain tube.

4. Water presence indicates the possibility of a clog. Look out for any water damage signs such as rust which indicates earlier water presence. Check your HVAC system’s water guide or post a question online to get help with clearing your clogged drip line.

Check your HVAC fuse

When it’s hot outside, your AC or heat pump works extra hard because your filters are clogged and your fuse is likely to burn out. When this happens, your Nest thermostat won’t get power supply from the HVAC system.

Steps

1. Turn off your breaker to cut off power to your HVAC system. If your system has multiple breakers, ensure your turn off all of them.
2. Check your fan and furnace and ensure no lights are still on which might indicate your system still has power.
3. On the HVAC system control board, look for the HVAC fuse. They come in different shapes. If you’re having trouble finding it, check with your owners’ manual.
4. Inspect the fuse. If it’s discolored or blackened, then it’s damaged and needs replacing.
5. After replacing, ensure to correctly reattach any panels you removed earlier to guarantee your furnace working properly.
6. Also ensure to turn on power at the breaker box.

If nothing still works after all these, then contact local support for help.

Conclusion

An E73 error can be annoying and has to be fixed before your Nest Thermostat works as usual. There are simple troubleshooting options such as checking your breaker box and ensuring the AC breaker is on. Others such as checking your drip pan/drain tubes can be complex and may need the services of an expert.

FAQs

Why is the meaning of E74 error on Nest Thermostat?

This means your Nest Thermostat is not detecting your Rh wire.

My Nest Thermostat is not won’t turn on.

This could be because of a power issue or your batteries being drained.