Nest Thermostat Turns On and Off in Cooling Mode Only

[3G]

Installed a Nest Thermostat in November for an oil heat and A/C system. A electrician added a ‘C’ wire.  The thermostat over the winter controlling the heat was without issue.  Today, I switched the thermostat to cooling mode. The A/C system turns on (both blower and compressor) for a moment and then off with the Nest flashing quickly “no power” with help code M20 before going into a delayed start countdown. The Wi-Fi also disconnects before reconnecting. When the countdown hits zero, the same thing occurs. I checked the wire connections and they are all secure. With the system off or in heating mode, the Nest indicates it’s getting power.  I confirmed by doing a power test. I put our old Honeywell thermostat back on to test the system and the A/C works fine. It’s therefore something with the Nest or power. I have a second system with a Nest on it too and I have the same issue; therefore, I don’t believe it’s the Nests themselves.  The bottom line is when the A/C turns on, there seems to be a lose of power to the Nest until it turns off moments later. Anybody have ideas on what the issue is?  

[Lm121212]

I am having the same problem.  Cooling worked fine last summer and nothing has changed.  The Nest is telling me to check the yellow wire, but it’s fine.  Can anyone tell me what I need to do?

[Jenelyn_O]

Hi there,
 

Thanks for reaching out. I'm sorry for the delay. I'll leave this support article for you to visit and check more information about the M20 code you received. Also, please check your Nest Thermostat wiring info by going to Settings and Equipment.

@Lm121212, you can visit the link as well and check your thermostat wiring info. 
 

Best,

Jenelyn

[Jenelyn_O]

Hello folks, 

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

Thanks,

Jenelyn

[3G]

The Nest is receiving power via the C wire. All other wires are securely connected. There is no issue with heat.  Everything works fine and did so since last fall. For some unknown reason, the Nest loses power as soon as the A/C turns on.  

[zoeuvre]

Hello everyone,

I wanted to stop by and see how I could help.

@3G, to troubleshoot a M20 error on your Nest Thermostat 2020, first is to make sure that the power to the system is on. The power switch is usually in the breaker box, but in some homes, it's like a light switch near the HVAC equipment. Then, check the thermostat wires. 

1. Remove the thermostat display.
2. Check all the connectors with wires are down.
   • If they aren’t, reinsert the wires:
     1. Press the connector button, and insert the wire as far as it goes.
     2. If the wire is in correctly, the button stays down when you release it.
   • The customer  might need to strip the wire ends. There should be ⅜ inch (9.5 mm) of straight and corrosion-free copper exposed.
3. Reinstall the thermostat display to its base.
4. Turn the power back on.

@Lm121212, I wanted to check and see if you managed to see the information that Jenelyn has shared. Please let me know if you have any questions from here. I would be happy to assist and make sure you are good to go.

Thanks for the help here, Jenelyn.

Best, 

Zoe

[zoeuvre]

Hi folks,

I wanted to check and see if you managed to see the information I shared. Please let me know if you have any questions from here. I would be happy to assist and make sure you are good to go.

Thanks,

Zoe 

[zoeuvre]

Hi everyone, 

We haven't heard from you in a while so we'll be locking this thread if there is no update within 24 hours. If you run into any more issues in the future, please feel free to create a new thread.
 

All the best,

Zoe