7 min read • Updated October 2026 • By the CliQ Team — the team that created Blink
Why is your Honeywell thermostat not working? Start with the screen. If it's blank, the thermostat has lost power: check the batteries, the circuit breaker, the furnace's power switch, the furnace door, and the condensate pump if you have one. If the screen is on but nothing runs, check that the mode and temperature are right, then wait five minutes, because Honeywell builds in a compressor delay. If settings keep changing back, it's usually the schedule doing its job.
This guide works through each symptom in the order to try the fixes, using Honeywell's own troubleshooting lists. Where a fix needs more detail, it links to the full how-to.
Find Your Symptom
| What you see | Most likely cause | Try this first |
|---|---|---|
| Blank screen | No power | Fresh batteries, breaker, furnace switch, furnace door, condensate pump (details) |
| "BATT," "Lo Batt," or "Replace Battery" | Low batteries | Change the battery |
| "Cool On" or "Heat On" flashing | Compressor protection delay | Wait 5 minutes without touching it (why) |
| Screen on, but no heat or AC | Wrong mode or setpoint, or a power interruption | Check the mode and setpoint, then wait 5 minutes (details) |
| Temperature changes back on its own | The schedule is running | Use Hold, or edit the schedule |
| Buttons won't change the temperature | Keypad lock or temperature limits | Unlock it, or check the allowed range (details) |
| Clock flashing or wrong | Power was interrupted | Set the time and day |
| Frozen or acting strange | Needs a restart | Restart it. Settings are kept |
Blank Screen: Five Power Checks
Honeywell's troubleshooting list for a blank display, from the T4 Pro and T6 Pro guides, covers the first four. The fifth is easy to miss.
- Batteries. Put in fresh alkaline batteries the right way round. The models in our battery guide take two AA or two AAA. Here's how, by model.
- Circuit breaker. Check the breaker for the furnace or air handler and reset it if it's tripped.
- Power switch. Make sure the switch for the heating and cooling system is on. It usually looks like a light switch, on or near the furnace.
- Furnace door. Make sure it's closed securely. Many furnaces won't run with the door loose. If you just changed the filter, check this first.
- Condensate pump. If your furnace or air handler has a small pump that carries water away, check whether it's full. Little Giant's owner's manual for its VCMA pumps says the high-water switch is configured to open a thermostat circuit when the reservoir fills. On some systems, that can leave a thermostat with no batteries completely dark.
Video by Chris McDowell via YouTube, June 2017. Not affiliated with CliQ.
His Honeywell VisionPRO IQ had no batteries to change, because it's wired to the house, and resetting the breaker didn't help. Then he found the condensate pump by the furnace full to the brim, with water underneath it. The outlet it plugs into had tripped, and its red light was on. One press of the outlet's button, the pump started draining, and the thermostat came back on. It's a two-minute check before you pay for a service call.
Screen On, but the Heat or AC Won't Run
Check the mode and setpoint
For heat, the mode must be Heat and the set temperature higher than the room temperature. For cooling, Cool and lower. Honeywell's troubleshooting lists put this first. Use the Mode button on a T4 or T6 Pro, or the System switch on an RTH. Setting guide here.
Wait five minutes
After a power outage, a battery change, or switching the system off and on, Honeywell thermostats make the compressor wait about five minutes. "Cool On" or "Heat On" flashes during the wait. Changing settings can start it over, so leave it alone. Full explanation.
Not working since you changed the batteries?
Check that each battery faces the right way. On a new unit, check that the paper battery tag is pulled out. Wait the five minutes. Then set the time and day: on RTH models, a flashing clock makes the thermostat follow Monday's Wake setting until you set it.
Still nothing? Run the breaker, power switch, and furnace door checks from the blank-screen list. Honeywell's guides repeat them for "heating or cooling system does not respond." If the furnace itself won't start, see furnace not turning on: check your thermostat first.
Heat pump running the wrong way? Honeywell's T6 Pro guide lists two symptoms, auxiliary heat running in cooling mode and cooling running when you call for heat. For both, it says to check the wiring on the wall plate. That's a wiring fix, and an HVAC technician is the right call if you're not comfortable with it.
Settings Won't Stick, or Won't Change
The temperature goes back on its own
That's the schedule. Pressing up or down only lasts until the next scheduled period. Press Hold to keep a temperature, or change the schedule. If it's the opposite and the thermostat is ignoring its schedule, see why a programmable thermostat stops following its schedule.
The buttons won't change the temperature
If the screen says "Locked" or "Unlock" on a T4 Pro or T6 Pro, press the center button and enter 1234, the code from Honeywell's user guides. Otherwise, check you're in the right mode, and that you're inside the allowed range: 40–90°F for heat and 50–99°F for cooling on the T6 Pro and RTH models. An installer can also set narrower limits.
It's frozen, or acting strange
Restart it: set the System to Off, pull the thermostat off the wall plate, remove the batteries for a minute, and put it back. Honeywell's T4 Pro and RTH manuals say settings are stored in permanent memory, so you won't lose your schedule. Restart, schedule reset, and factory reset, by model.
"Cool On" is solid, but the air isn't cold? Then the thermostat is doing its job, and the problem is the air conditioner. The US Department of Energy's list of common AC problems starts with a dirty filter, a clogged drain, and low or leaking refrigerant. The last one needs a trained technician.
Thinking about replacing your thermostat instead?
See the CliQ Smart Thermostat →Frequently Asked Questions
Why is my Honeywell thermostat not working?
Start with the screen. A blank screen means no power: check the batteries, breaker, furnace power switch, furnace door, and condensate pump. If the screen is on but nothing runs, check the mode and setpoint and wait five minutes for the compressor delay. If settings change back on their own, the schedule is running.
Why is my Honeywell thermostat screen blank?
It has lost power. Honeywell's troubleshooting list says to check the circuit breaker, the heating and cooling system's power switch, the furnace door, and that fresh AA alkaline batteries are installed correctly. If your system has a condensate pump, check that it isn't full. Its safety switch can cut power to the thermostat circuit.
Why is my Honeywell thermostat not working after I changed the batteries?
Check that the batteries face the right way and that any paper battery tag is removed. Then wait five minutes: Honeywell thermostats delay the compressor after a power interruption, and "Cool On" or "Heat On" flashes during the wait. Finally, set the time and day.
Why is my Honeywell thermostat not working after a power outage?
Usually it's the five-minute compressor delay, so wait it out without changing settings. Then check the clock, which may need resetting. Your schedule is kept on the T4 Pro and RTH models. If it's still off, check the breaker and the furnace's power switch.
Why won't my Honeywell thermostat turn on the heat?
Make sure the mode is Heat and the set temperature is above the room temperature. Then check the breaker, the furnace's power switch, and that the furnace door is closed securely. If the thermostat is calling for heat and the furnace still won't start, the problem may be the furnace itself.
Why won't my Honeywell thermostat turn on the AC?
Make sure the mode is Cool and the set temperature is below the room temperature, then wait five minutes for the compressor delay. If "Cool On" goes solid but there's no cold air, check the filter, the outdoor unit's breaker, and the condensate drain, then call a technician.
Sources
Resideo — T4 Pro Programmable Thermostat User Guide (33-00188EFS-11, 2022) — PDF
Resideo — T6 Pro Programmable Thermostat Installation Instructions (33-00181EFS-19, 2022) — resideo.com
Resideo — Programmable RTH2300/RTH221 Thermostat Series Owner's Manual — PDF
Honeywell Home — Why is the "Cool On," "Heat On," "Snowflake," or "Flame" blinking on the thermostat display? — honeywellhome.com
Little Giant — VCMA Condensate Pump Series Owner's Manual — PDF
US Department of Energy — Home Cooling 101 — energy.gov
Photo by Curtis Adams via Pexels
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