Still E74 "No power to "RH" Unusable Nest Thermostat G3

[JenkinsFamily]

This problem caused battery to die hard. It is taking way too long to charge up to maximum capacity. I can't do factory reset without full battery power. I know the wired connections are good. I checked the RH Connector wire and it has a steady 27.4VAC. There is no longer any reason why this is keeps faulting out. There seems to be some glitch that prohibits auto reset when corrections are made. So now I can't factory reset to manually clear this fault. What do Google Engineers say about these hundreds of exact same reports? No, Im really not happy about it and wish we would've purchased a more reliable user friendly product.

[MPT]

check the furnace filter. I had same issue this a.m. and changed the filter and error74 went away and furnace started heating

[MPT]

Now i'm getting T04 error. So I guess the filter didn't fix everything 

[JenkinsFamily]

I have a similar problem. I got a Nest Learning Thermostat Gen3 for Christmas. They told me it's easy enough for a DIY Homeowner to install. I followed the Google Nest App Installation instructions. It seemed to be working well at first. Then I added a remote temperature Sensor and set it to monitor an adjacent room during a certain time and the Nest all the other times of the day. That worked for a few days until apparently the remote temperature Sensor lost connection with Nest and my Heat would NOT turn off, it was 81 when I realized it was continuously running the Heat. To turn it off I went into Nest App settings and turned off that remote sensor. When that happens Nest reconnects with HVAC unit which was programmed to turn on A/C if temperature rises above 76. It quickly changed from Heat mode to A/C mode because of this. I don't think my Heatpump unit liked this very much. Since that happend I first got an E105 then eventually an E74. I can't continue from the E74 because it says "no power to RH". I looked at this connection and there is plenty of cooper tight in the connector. The voltage to the Thermostat is very low now, and that made it's battery die. I may have wired it wrong on the install or it may be something at the transformer. I'm still hoping to fix it myself before calling a Pro which will be big bucks. I attached pic of old therm/new therm wiring. Maybe someone can see my mistake or provide workaround tips. Thanks!!

 

[Jenelyn_O]

Hi there,

I'm sorry to hear you're having power issues with your thermostats — I'd love to know more about it. 

To narrow down:

• What is the make and model of your HVAC system?
• Remove the C wire and leave it disconnected. 
• Press and hold the thermostat for 10 secs  until it reboots.
• Wait until it turns on and check for any wiring report or message code. 
• If the same issue persists, check the power readouts below Technical  Info. 

I'll look forward to your response. Also, you can check out this guide.

Regards, 

Jenelyn

[Jenelyn_O]

Hi there,

I wanted to follow up and see if you still need help. Please let me know if you are still having any concerns or questions from here, as I would be happy to take a closer look and assist you further.

Regards,

Jenelyn

[MPT]

The E74 went away when I replaced the filter in the furnace but then I got a T04 which tells me nothing. Now I have errors on my furnace. I have had an HVAC contractor here for 2 visits and neither he or Carrier can determine the problem and they are going to replace the motherboard on the furnace but had to order it. Before the HVAC guy replaces the motherboard he is going to bypass the nest. I had to turn off the ECO and autoschedule because the furnace would not heat to the settings of the nest. Once I turned all the features off, the furnace started heating to the thermostat temp setting. I have had the nest for 6.5 years and the furnace for 2.5 years.

[Jenelyn_O]

Hi there,

I'm sorry to hear you're having power issues with your thermostats — I'd love to know more about it. 

To narrow down:

• What is the make and model of your HVAC system?
• Re-seat all the  wires into terminals.
• Remove the C wire and leave it disconnected.
• Press and hold the thermostat for 10 secs  until it reboots.
• Wait until it turns on and check for any wiring report or message code.
• If the same issue persists, check the power readouts below Technical  Info.
• Please share a photo of your thermostat wiring.

I'll look forward to your response.

Regards, 

Jenelyn

[MPT]

My furnace is Carrier model 58TN0A090C17-16. I'll have my HVAC person follow your Nest instructions and send it to you when he does but it won't be today. 

[Jenelyn_O]

Hey there,

Thanks for the information, @MPT, and I appreciate your effort in reaching out to an HVAC technician. Please keep us posted on how it goes. 

I would be glad to know what the results will be.

Regards,

Jenelyn

[MPT]

WE bypassed the Nest today and the furnace was still sending out errors. So the problem appears to be in the furnace. The furnace is working but is sending out faults and the exhaust fan is the suspect at this point. 

The Google Home app had indicated that the furnace had powered off 13 times in a 10 day period, which may have caused the E74 message on the nest. I did not get the Home app notification until 2 days after the E74 message and 12 days after the first power off. At any rate it does not appear to the be the Nest as the source of the E74, which I have not seen for a week now.

[JenkinsFamily]

*Latest Update: Thank you all for your helpful comments and insights into this problem. As a result, I have checked everything possible and rearranged every wire then back to the original configuration. No matter what was tried the persistent Error Fault E74 "No power to the RH connection on the Nest Learning Thermostat Gen 3" would not clear and allow the Thermostat to function properly. I contacted Google Support to get options and they again spent about 3-4 hours total trying everything again, finally they suggested trying my old thermostat. After I put back my old thermostat it worked perfectly. Google Support determined that I have a faulty BASE PLATE, the one where you connect and clamp down the wires on the Nest thermostat. Anyway, they want to send me a new BASE PLATE and try it and I return the defective one, free shipping. We will see what happens when I get that and reinstall everything. I will give you all an update after that happens. Thanks again everyone.

[Jenelyn_O]

Hey folks, 

That's good news, @MPT. Thanks for keeping me in the loop on what you've done to have your concern fixed. 

@JenkinsFamily it's good to hear that we had it replaced for you. I appreciate your time to check it with our support. Please keep us posted on how it goes. 

Regards,

Jenelyn

[MPT]

so the final fix was replacing the exhaust fan in the furnace, which kept shutting down and then the control board would reset.  I just wish the Home App/google next would have given me an earlier warning than it did.  10 days after the problem started is a bit late.

[Jenelyn_O]

Hi  there,

I appreciate your insight about this. The Nest Learning Thermostats would take time to learn your system to provide the correct code for a specific issue.

I suggest you send feedback about it from the Google Home app, here's how:

1. Open the Google Home app
2. At the top right, tap your account.
3. Tap Feedback

Regards,

Jenelyn

[JenkinsFamily · Accepted solution]

E74 Problem Solved. After troubleshooting the issue with Support it was determined to be a faulty base plate (which was brand new) we bought at COSTCO. Support FedExd a replacement. It was reinstalled. Happy to report it is error free and operating normally now and we're loving it. Just want to say thanks for all the help..

*This should be included in a solutions database for E74 possible causes.

[Jenelyn_O]

Hey JenkinsFamily,

Awesome, glad to hear! Thank you for keeping us updated. If you have any questions or concerns, please let us know as I will be locking this thread in 24 hours. 

Best,

Jenelyn