e79 common wire error code only with AC, not heat

[witsend]

This is the second summer in North Carolina with persistent e79 error code on Nest 3rd gen thermostat.  The system is gas forced air and AC.  With heat in the winter,  there is no e79 error code (which says no power to the C wire which charges the thermostat).  That code only appears when the AC is running.  The units are professionally serviced 2x/year.

The actions that have been taken that did not fix the problem:

     1.  new Nest 3rd gen thermostat installed ($150+)

     2.  completely re-wired from furnace/air handler to thermostat ( $250.  electrician, $950. HVAC tech)

Left alone, the thermostat will clear the code and re-start after minutes or hours.  This morning I pulled the top from the base and it re-started.  But I'm sure the code will re-appear because the top has been pulled many times during this process and the code always returns.  Has anyone experienced this or knows how to fix it?  Could it involve the condensate line?

Help.  I'm ready to dump nest.

[witsend]

additional info:  per the HVAC tech, the transformer is giving a clean 24 volts

[witsend]

Additional info:  the thermostat will return to normal operation if I reboot by depressing the top for ten seconds.  Then is runs normally.  This e79 error occurs multiple times per day, about 8 so far at 9:30 PM.  Is the unit just defective?

[humbertogarcia]

Hi witsend,

Thank you for posting in the community and for providing the issue information. I understand your frustration with the persistent E79 error on your Nest thermostat, especially since it only happens with AC and you've already invested in a new thermostat, rewiring, and professional servicing. This is a challenging situation, and I’ll be glad to work with you toward a solution.

Regarding your questions about if the condensate line could be involved - yes, absolutely, it remains a top suspect for intermittent, AC-only power issues. Even with regular service, a clog can form quickly, causing the safety switch to cut power to your AC circuit.

Please try this immediate check:

• Safely turn off power to your HVAC at the breaker.
• Locate your indoor unit's condensate drain pan/line. Check for standing water.
• If found, try safely clearing the drain line (e.g., using a shop vac on the outdoor end).
• Given the extensive troubleshooting already performed and the intermittent nature of the problem, this likely points to a subtle electrical issue that only manifests under AC load.

If clearing the condensate line doesn't resolve it, please contact your HVAC technician again. Emphasize that the E79 only appears when the AC is running, and ask him to specifically

• Monitor 24V AC voltage under load (while the AC is actively running).
• Thoroughly test the condensate float switch and other AC-specific safety components for intermittent faults.

Please let me know what you find; I’m here to assist further.

Regards,

Humberto

[witsend]

Humberto,

1). Are you AI?

2). As stated in my messages, it has already been verified (and repeatedly) that the thermostat is getting a "clean 24 volts")

3).  I messaged with the HVAC tech and he verified the the condensate pan and line have been checked repeated and they are clear

4). Do you understand the you were asking me to spend another $80 on a useless visit from HVAC tech (or to buy voltage testing equipment) when I have already spent over $1,300.00 on this issue????

Time to trash Nest???

[humbertogarcia]

Hi witsend,

Thank you for clarifying the information.

We're a dedicated team committed to providing the best support and clarifying questions related to Google Nest products for Google Nest users.
I understand the inconvenience the error message is causing and your annoyance with not yet having a solution. To help us correctly suggest the next steps and resolve the issue, I need some important information from you.

• Please provide a picture of your Nest Learning Thermostat (3rd gen) wiring. Also, check that all terminals are appropriately inserted.
• If possible and applicable, please attach a picture of your previous thermostat's wiring as well.

Additionally, please indicate when your thermostat was installed and who performed the installation.
I look forward to your response.

Regards,

Humberto

[witsend]

Installation was done by Delta HVAC in Raleigh  calldeltatoday.com , 

And I would love to send you a photo, but per this platform:  "You do not have permission to upload images."

[witsend]

Second thermostat installed for the second time after the new wire on June 27, 2025

[humbertogarcia]

Hi witsend,

Thank you for providing the details about the installation of your Nest Learning Thermostat (3rd Gen) units.

I understand you're receiving a "You do not have permission to upload images" message when trying to send the picture. To help you resolve this unexpected difficulty and successfully share your image, please try these steps:

1. Reply to this post to open the message editor.

2. Look for the camera icon (or similar attachment icon) in the toolbar above the text box.

3. Click the icon, and you'll likely be asked to drag and drop your picture into the designated area.

4. Please ensure your picture format is .JPG (or .PNG, as both are generally accepted).

5. Drag your picture into the upload area. If the file size exceeds the limit, you'll see a warning message.

6. Once the picture is uploaded, post your message.

If the above steps still don't work, please save the picture on your equipment or in your cloud and provide us the link in order for me to be able to access it.

I look forward to seeing the image so we can investigate further. Thank you for your continued cooperation.

Regards,

Humberto

[witsend]

Hi Humberto,

I couldn't find any camera/upload icon with which to upload an image directly, so I've uploaded it externally.

Here is the image: https://i.imgur.com/suOWP23.jpeg 

It shows professional re-wiring to the Nest as of June 27th.

[witsend]

Update:  the thermostat is shutting down much more often, and now displays an e74 code, no power to the Rh (heat) wire.  I pulled the top and am charging it from a cable to wall outlet.  

[humbertogarcia]

Hi witsend,

Thank you for the wiring picture and the update regarding the E74 error code. Your temporary measure to power the thermostat confirms it's a power delivery issue to the unit. Given the AC-specific and intermittent nature of the errors (E73/E74/E79), we need to check two common culprits.

1. Check Your HVAC System Fuse: A blown fuse can cut power to your thermostat.

• First, turn off all power to your HVAC system at the main breaker box. Ensure there are no lights on your furnace or air handler to confirm power is off.
• Locate the HVAC fuse, typically on the furnace control board. It's often a small, U-shaped (automotive-style) 3-amp fuse, sometimes pink or purple. (Refer to your owner's manual if needed).
• Remove and inspect the fuse for any signs of being burnt or discoloration. If it appears damaged, consult your HVAC system's user guide for replacement information.

2. Check for an energy savings box: Some utility companies install these boxes to manage power during peak times, which can cause these errors.

• Please check if there's an energy savings box near your electric meter or outdoor air conditioner.
• This is usually a small box with two lights (one red, one typically green) that allows your utility to cycle your AC on and off during specific peak hours.
• If you find one, wait an hour or two for the rush hour to pass, then check if the error disappears. If it does, please contact your utility company, landlord, or HOA for details about the program.

Please let me know the results of these checks. I'll be glad to assist you further.

Regards,

Humberto

[witsend]

No blown fuse.  The utility company savings box was removed about 4 years ago.  Enough.

I bought an Ecobee

[humbertogarcia]

Hi witsend,

Thanks for the update and for taking the time to troubleshoot with me. Because the reasons for the error codes received are not the ones that our troubleshooting steps suggest, the team would like to learn more about this behavior. When you get a chance, please fill out this form and let us know when you’re finished. We’ll have someone reach out to you via email from there.

Regards,

Humberto

[Scott321]

Does the Ecobee work better than the Nest.  I'm getting the error message and have tried everything.

[humbertogarcia]

Hi Scott321, 

Thanks for your post in the community! I'm truly sorry to hear you're experiencing an error message with your Nest Thermostat and that you're feeling frustrated after trying various solutions. I understand why you might be considering alternatives, but I'd really like to help you get your Nest Thermostat working reliably again.

To effectively troubleshoot this, I need a little more information about what's happening. Could you please tell me:

1. What specific Nest thermostat model do you have? (e.g., Nest Learning Thermostat 3rd Gen, Nest Thermostat E, or the newer Nest Thermostat 2020 model). This helps me provide the most accurate steps for your device.

2. What is the exact error message you're seeing on your thermostat's display or in the Nest/Google Home app? (Knowing the precise wording is very helpful for diagnosis.)

3. When you say you've "tried everything," could you briefly list some of the main troubleshooting steps you've already taken? (e.g., restarting your Wi-Fi, restarting the thermostat itself, checking wiring, etc.). This helps me avoid suggesting steps you've already completed.

Once I have these details, I'll be better able to investigate the issue and suggest specific solutions. I look forward to your response so we can work on this together.

Regards, 

Humberto

[Scott321]

it's a 3rd generation model.  The control says, "Wiring Report .... No power to Rh wire detected."

The fuses in the breaker box didn't trip.  The ac is four years old. I changed the air filter.  The ac drain pan is dry.  I restarted the control by holding it for 10 seconds and turned it back on.  When I recharge the control, the thermostat will work for about 20 minutes and then turn off.  My house has two ac units and the other one works fine.  When I switch controls, the same problem occurs at the same ac thermostat location, so it's not the control that can detach.  After charging the control, the fan will come on for the 20 minutes, but won't cool.  Note: the control is the round detachable thermostat.  I bought the house a year and a half ago and there hasn't been any problem since now.

[Scott321]

...after being off for about an hour and a half, it suddenly turned on and says "cooling" (the leaf or fan icon doesn't show).  Inside house temp. reads 86 and I have it set for 75.

[Scott321]

Now it cut off and went back to the same error message, "Wiring Report .... No power to Rh wire detected."

[Scott321]

Still not working

[Scott321]

still not working

[humbertogarcia]

Hi Scott321,

Thanks for your responses and for taking the time to provide me information. Because the reasons for the error codes received are not the ones that our troubleshooting steps could suggest, the team would like to learn more about this behavior. When you get a chance, please fill out this form and let us know when you’re finished. We’ll have someone reach out to you via email from there.

Regards,

Humberto