Thermostat Not Working: Fixes for a Blank Screen, No Heat, Wrong Readings, and Wi-Fi

A thermostat is a low-voltage switch that tells the furnace, air handler, or heat pump when to run. When it quits, the cause is usually power, a setting, placement, Wi-Fi, or wiring, and most of those are yours to fix.

This page goes symptom by symptom. If the thermostat is calling for heat and the furnace does nothing, see furnace not turning on. If you smell gas, don't touch the thermostat, light switches, or anything electrical, and don't use a phone inside: leave the house and call the gas utility or 911 from outside.

Most Likely Causes, by Symptom

The Screen Is Blank

1. Dead batteries. Honeywell's first step for a blank display is fresh batteries of the size marked in the compartment. On a wired model that also takes batteries, a screen that comes back on batteries means the thermostat is fine and the system is not sending it power.
2. The system has no power. A wired or smart thermostat runs on power from the furnace or air handler. Reset a tripped furnace breaker once, and if it trips again, leave it off and see our breaker page. Then check that the service switch is on and the blower door is firmly closed, since a loose panel trips a door switch that cuts power.
3. A blown fuse on the furnace control board. The thermostat circuit has a small fuse on the control board, usually 3 or 5 amps, per ecobee. It blows when wires touch with the power on or a wire shorts, and a new one blows again unless the short is found. Google says to have a professional check the system; it's the technician's.
4. A smart thermostat starved for power. Without a C-wire, a Nest charges its battery from the other wires; when that isn't enough, the screen goes dark, and Google's fix is a C-wire or its Nest Power Connector. ecobee thermostats need a C-wire or ecobee's Power Extender Kit; our C-wire guide covers the check. If an ecobee reboots during heating, ecobee's first suspect is the furnace overheating on its high-limit switch, and the first fix is a clean filter, per our filter guide.
5. The display is not seated. With the power off, push excess wire into the wall and press the thermostat onto its base until it clicks, as ecobee and Google describe.
6. The thermostat has failed. A battery-only model that stays blank with fresh batteries needs replacing, Honeywell says; for a wired one, it says to involve a technician. Before you replace it, see our smart thermostat guide.

The Screen Is On, but Nothing Runs

1. Mode and setpoint. HEAT with the setpoint a few degrees above room temperature, or COOL a few degrees below, per Honeywell's troubleshooting table. Older models may have a Heat, Off, Cool switch that is easy to bump. A fan set to ON instead of AUTO blows cool air between cycles; see furnace blowing cold air.
2. A schedule, hold, or away setting. A hold, an Eco temperature, or an away mode can keep the house at a setback temperature; clear it or resume the schedule.
3. A built-in delay. A Honeywell flashing Cool On or Heat On is protecting the compressor; Honeywell says to wait 5 minutes. Thermostats also wait until the room drifts slightly past the setpoint, 0.6 to 1°F by ecobee's default.
4. The furnace or air conditioner. Is the thermostat calling? An ecobee's flame icon turns orange or its snowflake blue, a Nest screen turns orange or blue, and many Honeywells show Heat On or Cool On. If it is calling and nothing happens, set the fan to ON. If the blower runs, the thermostat and power work, so the problem is the equipment; set the fan back to AUTO and see furnace not turning on. For no cooling, check the indoor unit's drain pan: ecobee notes a clogged drain trips a float switch that shuts the system off.

The Temperature Reading Is Wrong

1. A bad spot on the wall. ecobee lists the culprits: near a return vent, window, or door, or in direct sunlight. A supply register or lamp nearby makes it read warm. Moving it means extending wires, a technician's job; a remote room sensor is easier.
2. A draft from inside the wall. ecobee says the most common installation cause is the hole behind the thermostat letting wall-cavity air reach the sensor. With the power off, push excess wire in and seal the hole with plumber's putty, flat against the wall.
3. Dust, averaging, or a feels-like number. Honeywell says to keep a thermostat's air vents clean and unobstructed. An ecobee with room sensors shows their average, or a feels-like temperature if its humidity setting is on.
4. Calibration. Compare it with a reliable thermometer set beside it for an hour. ecobee rates its sensor at plus or minus 1°F and offers Temperature Correction of up to 10°F either way, but only for an error that stays the same; if it varies, the cause is placement.

The System Short Cycles

Check the thermostat side first. A thermostat above a supply register or in the sun is satisfied too soon. Google lists rapid on-off cycling, chattering from the system, and a Delayed message among signs a Nest may need a C-wire or its Power Connector. The cycle rate must match the equipment: Honeywell has different heating cycle rates for standard, high-efficiency, and electric furnaces, and ecobee says to contact its support before changing its thresholds. With the power off, tug each wire and reseat any that move. Then see furnace short cycling.

A Smart Thermostat Shows Offline

An offline Nest still works from its own screen, Google notes, and a blank screen is power, not Wi-Fi.

1. Check the internet on another device, then unplug the modem and router for 30 seconds, as Google describes.
2. Restart the thermostat: an ecobee off the wall for 30 seconds, a Honeywell Wi-Fi model off its wallplate for 10, or a Nest Learning Thermostat's Restart option. After a new router or password, reconnect it.
3. Check the signal and power. ecobee says strength below 75 percent can cause dropouts, and some models need a 2.4 GHz network; a Nest low on battery stops using Wi-Fi, Google says. ENERGY STAR advises keeping the thermostat reasonably close to the router or adding a range extender.
4. Try a phone hotspot, as ecobee and Google suggest. If it connects, the problem is your home network's settings; if not, call the maker.

It Stopped Working After a New Install

With the power off at the breaker or service switch, as all three makers instruct:

• Match the photo, by letter. Match your photo of the old wiring by terminal letters, not colors. Google says wire ends should be straight, stripped 3/8 inch, and pushed in until the connector button stays down.
• Heat when you want cooling, or the reverse. On a heat pump, suspect the O/B reversing valve setting; ecobee notes most heat pumps energize it in cooling (O). Switch O to B, or B to O, and test both modes, as ecobee and Google describe, and confirm the setup lists a heat pump.
• The old-thermostat test. Google's test: reinstall the old thermostat. If the system works, the new one has a wiring or power problem; if not, call a technician.

Line-voltage thermostats are a different, dangerous category. Baseboard and wall heaters often use a 120- or 240-volt thermostat, which Google describes as typically two to four thick wires, often joined with wire nuts, and a label reading 110, 115, 120, or 240 VAC. Standard 24-volt thermostats, smart ones included, don't work with them, Google and Honeywell say. Honeywell's line-voltage thermostat warns that mishandling it can cause serious injury or death. Never touch that wiring with the power on; call an electrician.

If the old thermostat has mercury. Older dial thermostats often switch with a sealed glass tube of mercury. EPA says they rarely leak in use but must be disposed of properly: take the old one to a household hazardous waste collection site for recycling, never the trash, sealed in a container with a tight-fitting lid and secured so it can't tip on the way. Some states, Vermont among them, ban household mercury waste from landfills. The Thermostat Recycling Corporation lists drop-off sites.

What You Can Fix Yourself

• Replace the batteries; check the mode, setpoint, fan, schedule, and holds.
• Reset a tripped breaker once, turn on the service switch, close the blower door.
• With the power off, reseat the thermostat, fix loose wires, and seal the wall hole.
• Restart the router and thermostat; correct the heat pump O/B setting.
• Recycle a mercury thermostat at a collection site.

When to Call a Pro

Call an HVAC technician when the control-board fuse has blown, the breaker trips twice, the system fails with the old thermostat too, you need a C-wire or Power Extender Kit connected at the furnace and would rather not open it, the thermostat must move, or the wiring matches nothing in the maker's app. Honeywell warns that improper wiring or setup can cause property damage or injury. Numbered terminals or more than eight wires usually mean a communicating or zoned system, also the technician's. Line-voltage thermostats go to an electrician.

Prevent It

Replace thermostat batteries yearly. Check the furnace filter monthly and change it every one to three months; a clogged filter can shut down the furnace and a smart thermostat with it. ENERGY STAR's annual tune-up checklist includes the thermostat settings. When you travel in winter, keep the heat no lower than 55°F and have someone check the house.

Frequently Asked Questions

Why is my thermostat blank after I changed the batteries?

Check that they face the right way. On a wired model, check the breaker, service switch, and blower door; if it still won't power up, Honeywell says to call a technician.

How do I reset my thermostat?

Restart it first, which keeps your settings: off the base for 30 seconds (ecobee) or 10 seconds (Honeywell Wi-Fi), or a Nest Learning Thermostat's Restart option. A factory reset erases the schedule and setup; save it for when the maker's support asks.

Can I replace a baseboard heater thermostat myself?

Not with these steps. Baseboard thermostats usually run on 120 or 240 volts, which makes the swap house wiring. Hire an electrician.

The Bottom Line

Batteries, breaker, service switch, blower door, then mode, setpoint, schedule, and hold. If it's calling and nothing runs, look at the furnace or air conditioner. Wrong readings are usually placement, offline is usually the network, and blank is usually power. Power off before touching a wire, leave blown fuses and line-voltage thermostats to a pro, and recycle mercury thermostats.

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