temperature reading fluctuate high and low

[MingZ4]

Have this thermostat for several years, so far so good. I use an external power adapter for c-wire. A few days ago, the temperature reading go high to 90F when other thermometers read ~72F.  The AC is triggered, and temperature drops quickly to ~ 70F in 5 min, while the other thermometers drops to 71F.  This temperature fluctuation keeps going. I restarted the nest a few times, it did not work. then did a factory reset, replaced with the brand new batteries, measured the external c-wire power supply which is good,  the issue remains.    

does not see any issue in google home app. The SW version is: 2.2-6, model 1.4

[kcruzgonzalez]

Hi @MingZ4,

Thank you for posting in the community. I understand that your Nest Thermostat is experiencing inaccurate temperature readings after several years of working perfectly. It sounds incredibly challenging when a device that controls your home's comfort suddenly starts to behave unexpectedly, especially after you've already completed such thorough troubleshooting. Don't worry, I'm here to help.

I understand that you have already restarted the thermostat, performed a factory reset, replaced the batteries, and confirmed that your external C-wire power supply is working correctly. To help me get a better understanding of the issue, could you please provide some more details?

• Could you please confirm the specific model of your thermostat? (Please visit our Help Center to learn more about how to tell which Nest thermostat you have).
• Do you have any schedules or routines set up in the Google Home app that might be affecting the thermostat's temperature settings?
• Is the thermostat located near a heat source, a window with direct sunlight, or an air vent that might cause the sensor to read an incorrect temperature?
• Do you have any other Nest temperature sensors in your home, and if so, what are their readings?
• When did this fluctuation in temperature readings first begin? Was it after a specific event, like a recent software update?

Once I have a better understanding of the issue, I can provide the right support. I look forward to your response.

Regards,

Kevin

[MingZ4]

Hi Kevin, 

Model: Thermostat 1.4.  It does not support temp sensor.

I have schedule, and have been using the scheduling since I installed it, ~5 yrs ago.

It is not near heat source or window or vent. Until recently, it has been working well.

I have several temperature sensors, Govee and ThermoPro, their reading are pretty close, within 1 F among each other, in the range of 60~80F, nowhere near 90F.   

The issue happens a few days before my initial post. no specific event, I unplugged then plugged c-wire adapter, but I have done that occasionally before. I measured the voltage of adapter output, ~24V, as specified. 

Thanks

Ming

[kcruzgonzalez]

Hello @MingZ4,

Thank you for contacting us again regarding your Nest Thermostat. Thanks for answering the questions, and I'd be happy to continue helping you with this.

Based on the issues you've described, a simple restart can sometimes recalibrate the sensors. While you've already tried this, it is part of the recommended troubleshooting flow to try again before a setting change.

To restart your thermostat:

• On the thermostat’s display, go to Settings.
• Select Restart and confirm by selecting Restart again.

The next step is to ensure that a feature called Sunblock is enabled. Direct sunlight can cause the Nest thermostat's temperature sensor to heat up, which would explain the high readings you are seeing.

Please follow these steps to turn on Sunblock:

• Open the Google Home app on your phone.
• Select the thermostat.
• Tap Settings, then Temperature Preferences.
• Turn the Sunblock feature on.

Please let me know if these steps help resolve the issue. I look forward to your response.

Regards,

Kevin

[MingZ4]

Restart does not work.

There is no Sunblock option in Google home app, Temperature preference

Thanks

[MingZ4]

replaced the thermostat, new one has been working so far.  please close this case. thanks