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ErrBase

Nest Thermostat Error Codes: E73, E74, E298, W5

Quick Answer

Nest thermostats (Nest Learning Thermostat, Nest Thermostat E, and Nest Thermostat 2020) display error codes in the Home app and on the device screen. E73 means the thermostat sent a cooling signal but no power was detected on the Y1 wire - the air conditioner did not turn on.

I've been on probably a hundred of these Nest calls over the years, and the thing I always tell homeowners is that the Nest itself is almost never broken. It's just the messenger. E73, E74, E298, W5, these codes mean your HVAC system has a problem somewhere and the Nest is smart enough to tell you about it. Ignore them long enough and you're looking at frozen coils, a flooded drain pan, or a furnace that won't start on the coldest night of the year.

NestThermostat

About These Nest Thermostat Error Codes

When your Nest throws a code like E298 or W5, it's often telling you it's starving for power or waiting on equipment that isn't responding. I see this most in homes without a dedicated C-wire where the thermostat tries to pulse the HVAC system for a trickle charge. If your AC or furnace was working fine until a heat wave or cold snap hit and the system started running constantly, that's usually what tips it over the edge.

Most Common Error Codes

HVAC system not responding to thermostat signal (E73/E74)40%
No C-wire providing steady power (E298)24%
Blown HVAC fuse or tripped breaker14%
Wi-Fi router changed or moved (N260)12%
Contactor or relay failed in outdoor unit10%

Symptoms You May Notice

  • Error code shows up in the Google Home app with a timestamp, sometimes showing the exact minute the system stopped responding, which is actually super useful for figuring out what triggered it.
  • You set the AC to 72 and the house just doesn't cool. The Nest display shows it's calling for cooling with the leaf icon but nothing happens outside, and the outdoor unit is completely silent.
  • The furnace fan kicks on and you can hear air moving, but what's coming out of the vents is room temperature or cold air, meaning the heat exchanger never lit.
  • Thermostat display randomly goes blank for a few seconds and comes back, especially during long cooling runs in the summer, which is the Nest's battery browning out from a C-wire power issue.
  • System short-cycles constantly, turning on for two or three minutes then shutting off before it ever reaches the setpoint, then starting again a few minutes later over and over all night.

Tools Required for Diagnosis

Phillips #2 screwdriverSmall flathead screwdriver (for prying wire terminals)Multimeter with AC voltage settingFlashlight or headlampNeedle-nose pliers (for seating wire terminals)

How to Identify Your Error Code

ComponentComponent Under Test
Expected Range24–28 VAC
ConditionIf Open (OL) or infinite, replace component.

Replacement Parts

If your diagnostic testing proves the component has failed, you will need a replacement. We recommend OEM parts over aftermarket for water-handling components.

Part Name
Nest C-Wire AdapterGoogle C-wire adapter · $25–$40
Nest Thermostat Trim KitGoogle · $15–$20

Frequently Asked Questions

E73 on Nest but my AC was working yesterday. What happened?
E73 almost always pops up because the outdoor unit lost power, not because the Nest failed. I find a tripped float switch from a clogged condensate drain line probably 60% of the time on these calls. Check your drain pan under the indoor coil for standing water first. If it's dry, go find the breaker for your outdoor condenser unit, usually a double-pole breaker in the main panel labeled AC or COOL. Also check the disconnect box right next to the outdoor unit, those get bumped all the time. It's rarely a Nest problem. The Nest is just reporting what it sees on the Y wire.
E298 keeps coming back. Do I need a new thermostat?
You don't need a new thermostat. E298 is almost always a C-wire problem. Without a dedicated common wire, the Nest has to steal tiny amounts of power from the heating or cooling circuit while the system is off, and on some modern variable-speed systems that technique just doesn't work reliably. The Google Nest Power Connector kit is about $15 and fixes this permanently without running new wire in most cases. I've installed probably 40 of those things in the last couple years and haven't had a single callback on one. Way cheaper than a new thermostat.
How do I check Nest error history?
Open the Google Home app, tap your thermostat, go to Settings then Pro Setup, and look for Error History. It'll show you every code with a timestamp. On the thermostat itself, turn the ring to Settings, then Equipment, then Test. That runs a live diagnostic and tells you whether voltage is actually present on each wire right now. It's super useful for figuring out if your E73 is from a dead Y wire or from a relay issue in the outdoor unit. Always check the history before you start pulling wires out of the base.
W5 but my furnace has no error lights. What's wrong?
W5 is the Nest saying it's waiting on the furnace to respond. No error lights on the board actually means the board might not be getting power at all, so check the furnace door interlock switch first. Also go into the Google Home app and run an Equipment Test to confirm the W1 wire is delivering signal. Then check for a clogged air filter, honestly I've seen brand new high-efficiency furnaces lock out from a dirty filter after just four or five months. If the filter's clean and the door's seated, you might have a gas supply issue or a failed hot surface igniter. Those run about $30 to $50 and they're a pretty common failure on older furnaces.
Can I just put my old thermostat back on while I figure this out?
Yes, absolutely, and honestly that's a smart move if you need heat or AC right now. Take a picture of the wires in the Nest base before you disconnect anything so you know which color goes to which terminal. Your old thermostat will work fine on the same wires. The only thing to watch for is if your old thermostat ran on batteries, make sure they're fresh. Most conventional thermostats don't even use the C-wire, so if the Nest was struggling without one, the old thermostat won't care at all. It's a completely safe way to keep your system running while you troubleshoot.

Models Known to Experience HUB Errors

This repair applies to most Nest thermostats with this error code. Common model numbers include:

T3007ES (Nest Learning Thermostat 3rd Gen), T3008US (Nest Learning Thermostat 3rd Gen, alternate SKU), T4000ES (Nest Thermostat E), T4001ES (Nest Thermostat E, alternate SKU), T3010US (Nest Thermostat 2020), GA02081-US (Nest Thermostat 2020, alternate SKU), GA03815-US (Nest Learning Thermostat 4th Gen)

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Written by

Raj Patel

HVAC & Water Systems Specialist · 15 years experience

Last verified for technical accuracy on March 15, 2026