E73 Error shows up out if nowhere and only at certain times

[Alanahr5]

So about a week ago it got down to 35°F and all of a sudden we got a heads up alert on the nest app turns out our 1 year old Nest Learning started throwing an E73 error code after 12-15 minutes of heating on each cycle between 10pm-6am outside of those times it works fine. It also worked as normal prior to a week ago no issues no errors on the thermostat. The furnace blower motor will still run but the fan shuts off and the thermostat throws an E73 error after about 1-3 minutes the fan and thermostat comes back up. We have installed a common wire which did not fix the issue. We are currently near -30°F and I can't turn my temp up (running thermostat between 66°F and 67°F) worried it will be worse. Furnace itself is NOT showing any error codes. All components except for the inducer motor, blower motor and limit switches have been replaced in the furnace. Our furnace is pretty basic it is a Nordyne Intertherm 

[zoeuvre]

Hi Alanahr5, 

Uh-oh! The E73 error code on a Nest Thermostat typically indicates a communication issue between the thermostat and the heating system. A few questions: 

1. Make sure the breakers are on. 
2. If you used AC recently, check your system’s drip pan or drain tubes (skip this step if you only have a heating system). 
3. Check the condition of the HVAC fuse.
   • Turn off the main power to the HVAC system at the breaker box. Note: The HVAC system can have multiple breakers, so make sure the customer turns them all off.
   • Check the furnace and fan and make sure there are no lights that might indicate the system still has power. Note: If there are still lights on the furnace or fan, the system might still have power, and you need to turn off the main power to their home at the breaker.
   • Search for the HVAC fuse. It’s usually on the furnace control board. Note: If you need help to find their control board, check your owner’s manual. Fuses can be different for each type of system. You might find a small U-shaped fuse with a number 3 label and is pink or purple colored. You might also find a round fuse. 
   • Remove and inspect the fuse for damage. If the fuse seems burnt or has discoloration, consider a fuse replacement. To learn more about fuse replacement, consult the user guide for the HVAC system or contact a Nest Pro.
   • Make sure to reattach any panels on the furnace. Note: The furnace might not work if the customer doesn't reattach the panels.
   • If you're not comfortable checking the HVAC fuse, the replacement fuse doesn’t fix the issue, or the you can’t find the fuse, proceed to the next step.
4. Check you have an energy savings box. Some utility companies install boxes that cut off the power during high-demand (peak) times. These energy savings boxes can cause the error.
   
   • Check if there’s an energy savings box in your house.
     1. Search for an energy savings box near the electric meter (possibly near their air conditioner).
     2. Search for a small box with 2 lights on it (one light is red, the other is usually green). This box allows the utility company to cycle the your air conditioner off and on during certain peak times of the day.
   • If you have an energy savings box:
     1. Wait an hour or two for the rush hour to end, and then check if the error disappears.
     2. If the error disappears, contact their utility company, landlord, and/or Homeowners Association (HOA) for details about the program and your home's participation in it.
   • If you doesn’t have an energy savings box, proceed to the next step.
5. Check your air filter. You HVAC system needs to have a good airflow to work properly. A clog in the air filter can limit how much air flows over the cooling coils. If the clog persists, the cooling coils can freeze and cause your system to shut off.
6. Find your HVAC system’s air filter. The air filter is usually behind a grate in the hallway.
   • Check your walls and ceiling.
   • If you can’t find the air filter, search for it inside the furnace near the fan.
7. If the air filter seems dirty or there seems to be a clog, replace the filter with a new one. If the issue is because of frozen cooling coils, you need to wait a while for the HVAC system to thaw before they can use it again.

Keep us posted. 

Best,

Zoe

[Alanahr5]

It was actually the thermostat itself the Rc terminal went bad. Had to replace the entire thermostat. Sucks spent almost $300 at Walmart in October 2022 for it. We have an old basic Nordyne Intertherm and basic old school ac we don't have all of the fancy stuff and our air filter is a hog hair on the door panel that comes off to get to the inside of the furnace 

[zoeuvre]

Hi there, 

Thanks for getting back to us, and I'm sorry for the late response. We're sad to see you go. We're always here to help if you ever change your mind. If you have questions or concerns in the future, feel free to start a new thread, and we'll be happy to help.

Regards,

Zoe