my nest thermostat e just went blank and won't turn on, no lights or anything, what do i do?

Diagnosis

The Nest Thermostat E has completely lost power, meaning the internal battery has depleted and it is no longer receiving a 24VAC charge from your HVAC system. Unlike a software glitch where the screen might freeze or show an error code (like an E-series power error), a total blackout indicates a break in the electrical circuit between the thermostat base and the furnace/air handler. This typically happens for one of three reasons. First, a tripped float switch in your AC condensate pan has cut power to the R-wire to prevent your house from flooding. Second, the 3-amp or 5-amp blade fuse on the furnace control board has blown due to a power surge or a short in the wiring. Third, if you do not have a "C" (Common) wire installed, the thermostat was "power stealing" from the Rh/Rc wire; if the battery level dropped too low during a period of inactivity, the device cannot trigger the system to wake up. You can confirm this by pulling the display off the base and plugging it into a micro-USB cable for 30 minutes; if it boots up, the issue is definitively with your HVAC power delivery.

How to Fix It

Follow these steps in order to restore power to your Nest Thermostat E:

1. Check the HVAC Float Switch: Locate your indoor AC unit. Look for a PVC drain pipe with a small switch (the float switch) attached to it. If the secondary drain pan is full of water, the switch will trigger and kill power to the thermostat. Use a wet/dry vacuum to suck out the clog from the exterior drain line to empty the pan and reset the switch.
2. Inspect the Furnace Fuse: Shut off the main power switch to your furnace (usually a toggle switch on the side of the unit). Open the blower compartment panel. Locate the control board and look for a small, automotive-style blade fuse (usually purple 3A or orange 5A). Pull the fuse and inspect the filament; if it is broken or charred, replace it with an identical amp-rated fuse from a hardware store.
3. Verify the Breaker: Go to your main electrical panel. Do not just look for a switch that is "Off"; look for one that is slightly out of alignment (tripped). Flip the HVAC/Furnace breaker completely OFF and then back to ON.
4. Perform a Manual USB Jumpstart: Remove the Nest display from the wall base. Locate the micro-USB port on the back of the device. Plug it into a high-output wall charger (at least 5V/1A) for 60 minutes. This bypasses the HVAC system and forces the internal lithium-ion battery to a charge level high enough to reconnect with the base.
5. Test the Base Connection: Once the display is charged, press it firmly back onto the base. Ensure the connector pin is not bent and that the wires are seated deeply in the terminals, specifically the Rh and C wires.

If That Didn't Work

• Install a Nest Power Connector: If you have a 4-wire system without a C-wire, the "power stealing" method often fails over time. Install the official Nest Power Connector (part number GS0030) at the furnace control board. This provides a dedicated return path for electricity, preventing the battery from draining to the point of a blackout.
• Test for 24VAC with a Multimeter: Set your multimeter to AC voltage. Touch the probes to the Rh and C wires at the thermostat base. If you read less than 24V, the issue is a broken wire in the wall or a failed transformer inside the HVAC unit.
• Inspect the Air Filter: A severely clogged air filter can cause the furnace to overheat, triggering a "limit switch" that shuts down the entire system, including power to the thermostat. Replace your filter and wait 30 minutes for the limit switch to cool down and reset.
• Replace the Base Plate: If the display works via USB but never powers on when attached to the wall (and you have verified 24V is present), the base plate's internal circuitry may have failed. Replace the base plate while retaining your existing display.

Heads Up

• Avoid Low-Power USB Ports: When jumpstarting your Nest E, do not use a computer USB port; they often provide insufficient amperage to wake a deeply discharged battery. Use a dedicated wall brick.
• C-Wire Necessity: While the Nest E is marketed as compatible without a C-wire, in colder climates or with high-efficiency systems, a C-wire is practically mandatory to prevent intermittent power cycles.
• Fuse Warning: Never replace a 3A fuse with a 5A or 10A fuse. Doing so protects the circuit less and could lead to the furnace control board burning out completely.
• Condensate Maintenance: If your thermostat went blank due to a float switch, add a drop of dish soap or a specialized algae tablet to your condensate line every six months to prevent future clogs.