topic Nest Gen 4 will not work, but Gen 2 does in Home Automation

Nest Gen 4 will not work, but Gen 2 does

ehs1331 — Mon, 17 Nov 2025 11:56:51 GMT

Subject: Nest Gen 4 Setup Issue – RH/RC Voltage Error (E402)

Hello,

I’ve been troubleshooting an issue with my Nest thermostat but haven’t found a solution. My Nest Gen 2 worked flawlessly for years, but since the app was discontinued I had to upgrade to the Gen 4.

The wiring plate on the Gen 4 is identical to the Gen 2, with the same terminal locations. However, during setup in the Google Home app, the RH and RC terminals fail and show no voltage, resulting in error code E402.

Here are the details:

My level light is on.
Voltage tested at ~27 volts.
Y1, G, and W1 all passed successfully.
Reinstalling the Gen 2 works perfectly with the same wiring.
I’ve inspected the wiring and terminals multiple times.
I’ve included a photo of my Gen 2 wiring for reference.

Any guidance or suggestions would be greatly appreciated.

Thank you,
Eric Stevenson

Re: Nest Gen 4 will not work, but Gen 2 does

Patrick_Caezza — Mon, 17 Nov 2025 23:34:41 GMT

@ehs1331

First, you can not put two wires into one terminal.

The jumper between the Rc and Rh terminals is not needed. Connect the red wire to the Rc terminal. The Nest will automatically jump the two together.

The white and black wires need to be separated. Take a look at the furnace and see what they are connected to.

Re: Nest Gen 4 will not work, but Gen 2 does

ehs1331 — Thu, 20 Nov 2025 18:34:39 GMT

thanks for the quick reply. thats how they connected it when the gen 2 was installed and seem to work fine for years. I will try to figure out what the 2 wire are. could that be why the app is showing no voltage to Rc? I'm out of town and will try the change this weekend.

Thanks again,

Eric