topic Re: phantom OB wire in Nest Thermostats

phantom OB wire

OBWire — Mon, 02 Mar 2026 07:58:43 GMT

This has been an issue since the Nest was first installed a few months ago.

It thinks there is an OB wire when there is none.

I have a regular gas furnace heater and an air conditioner (not a heat pump).

I have inserted Y1, G, W1, C, Rh.

A few days after install, the thermostat stopped responding appropriately (turns on AC no matter if HEAT or COOL was selected) and I found that it sensed a phantom OB wire.

I was able to fix it by simply toggling the little OB connector (never inserted any wire) and then it stopped detecting the OB wire.

This fix worked for months. However, a few days ago, it suddenly started detecting OB again and I have not been able to fix it since (even after FACTORY RESET and re-inserting all wires).

Images attached.

https://imgur.com/a/jF2h2xs

https://imgur.com/wf19fsr

Thanks in advance

Re: phantom OB wire

StephanieQ — Sat, 07 Mar 2026 16:01:39 GMT

Hi @OBWire,

Thank you for posting in the community. I’m sorry to hear about the trouble you’re experiencing since the installation of your Nest Learning Thermostat (4th gen); it appears the system is detecting an OB wire despite no physical wire being connected to that terminal. I also see that a few days after installation, the thermostat began turning on the AC regardless of whether Heat or Cool was selected. I appreciate the pictures and details you provided regarding your wiring setup and HVAC system, as well as the troubleshooting steps you've already taken. I understand how frustrating this situation could be for you, and I’m here to help get your thermostat working properly again.

To help me review your setup, could you provide a clear photo of your HVAC control board wires? You can refer to this article for detailed instructions on how to Check your system’s control board.

In the meantime, I recommend trying these next steps:

Open the Google Home app.
Tap Home > Climate > select your thermostat.
Tap Settings > Thermostat > Wiring, and then Update Wiring or Wire Configuration. If the OB terminal shows as selected, manually deselect it and save your changes.

I look forward to your reply so I can assist you further.

Best regards,

Stephanie

Re: phantom OB wire

OBWire — Fri, 06 Mar 2026 18:20:04 GMT

There is no "update wiring or wire configuration" setting in the app.

Update: I ended up replacing the baseplate and that was the only solution that worked. My original baseplate is physically defective.

Re: phantom OB wire

StephanieQ — Sat, 07 Mar 2026 15:52:24 GMT

Hi @OBWire,

Thank you for the update and for confirming that replacing the base resolved the issue; it appears the original baseplate was defective. I also appreciate your letting me know that the "Update Wiring or Wire Configuration" setting isn't appearing in your app.

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.

To help us assist you better, please include the link to your Community thread and your Community username.

Keep me updated.

Best regards,

Stephanie

Re: phantom OB wire

StephanieQ — Mon, 09 Mar 2026 18:17:39 GMT

Hi @OBWire,

We haven’t seen your form submission come through - were you able to access it alright? Let us know if you’re running into trouble or still need our help!

Keep me posted.

Regards,

Stephanie