Honeywell Thermostat Not Working? What to Check First

Start with what the screen shows

When the house won't warm up or cool down, the thermostat's screen is your first clue. We'd read it before touching anything else or shopping for a new thermostat, because a blank screen, a flashing message and a normal-looking screen each point somewhere different.

A blank screen means the thermostat isn't powering up, so start with its power source. On a battery-powered model, that means fresh batteries. A wired model gets its power from your heating and cooling system, so Honeywell's checks for a screen that won't power up (opens in a new tab) start at the furnace or air handler.

Find that equipment's power switch, which can look like an ordinary light switch, and make sure it's on. Its door or cover should be fully closed too, since some systems won't power up otherwise. If the door is open or loose, book an HVAC technician. If the equipment's breaker has tripped, call a licensed electrician.

If the screen is on, set the mode to Heat with the target above the room temperature, or to Cool with the target below it. A flashing icon or message, or a Wait message, means the compressor protection delay (opens in a new tab) is running. Honeywell says it can last up to five minutes and keeps the equipment from coming on too quickly, which can damage it.

The step-by-step fixes below are for two battery-powered models, the RTH2300 and RTH221, so check your paperwork to confirm which model you have. For any other Honeywell thermostat, find your model's manual with Honeywell Home's manual lookup (opens in a new tab), because batteries, power and buttons vary from model to model.

Fixing a blank RTH2300 or RTH221 screen

Look closely first. On the RTH2300, a dark backlight with numbers you can still read isn't a blank screen. If Lo Batt has been flashing, that was your warning, since it shows for 60 days before the batteries run out. For a truly blank screen, follow the battery steps for these two models (opens in a new tab) with two fresh AAA alkaline batteries.

1. Set the System switch to Off before you take the batteries out. Otherwise the heating or cooling can stay on while they're out.
2. Pull the thermostat body away from the wallplate at its bottom edge. Leave the wallplate on the wall and keep your hands off the wires. If the body won't come off that way, stop.
3. Turn the body over and check that the model number on the back says RTH2300 or RTH221 (for any other model, follow its own manual). Put in two fresh AAA alkaline batteries, matching the polarity marks, then put the body back on the wallplate.
4. If the numbers come back, press Set and use the up or down arrow to set the clock. On the RTH2300, press Set again and choose the day. Press Run to save and exit. Your other settings stay saved.

If the screen stays blank with the batteries in correctly, call Honeywell Home technical support (the number is at the end). For the RTH2300, Honeywell's support answer (opens in a new tab) is to replace the thermostat. If the numbers come back, keep going with the next check before you call it fixed.

To keep this from happening again, replace the batteries once a year, or before you'll be away for more than a month.

The RTH2300 or RTH221 screen works but nothing comes on

On these models, the big number is normally the room temperature, and pressing an arrow shows your target for five seconds. A running fan only tells you the fan works, so feel for real heating or cooling at the vents.

With these two models, don't run the air conditioning when it's below 50°F (10°C) outside, because that can damage the compressor. Leave the compressor protection alone too, since it's there to prevent equipment damage.

1. Use the System switch at the bottom to pick Heat or Cool, and set the Fan switch to Auto. With the fan set to On, it can run nonstop without any call for heating or cooling.
2. Use the arrow buttons to the right of the display to set the target above the room temperature for heat, or below it for cooling.
3. Wait five minutes without switching modes back and forth. Flashing Cool On, or flashing Heat On on a heat pump system, means compressor protection is running, so let the delay finish.
4. Check that the equipment starts and actually heats or cools. If it keeps flashing, doesn't respond, or only moves air without heating or cooling, book an HVAC technician.

Five minutes shows you whether the system responds, and reaching the target temperature can take longer. If the trouble started after the thermostat was replaced, tell the installer. The RTH2300 and RTH221 don't work with heat pumps that have auxiliary or backup heat (opens in a new tab), and changing the target won't fix that.

Your RTH2300 or RTH221 keeps changing the temperature

Pressing an arrow only overrides the schedule until the next programmed period starts, and Temporary shows on the display while it does. That's why a thermostat that's working fine can seem to ignore your change later on. We'd use Hold whenever you want a temperature to stay put.

1. Press the up or down arrow until the temperature you want is showing.
2. Press Hold. Hold appears on the screen, and that temperature stays until you change it or press Run.
3. Press Run when you want the programmed schedule to take over again.

If the schedule switches at the wrong time of day, check that the clock is right, and on the RTH2300 the day as well. Honeywell's schedule override steps for Series 2000 thermostats (opens in a new tab) use these same Hold and Run buttons, and none of this needs the wiring or installer settings touched.

Who to call if it's still not working

If the screen stays blank with fresh batteries, call Honeywell Home technical support at 1-800-633-3991. On another model, you can also start from Honeywell Home's troubleshooting selector (opens in a new tab). If the screen works and calls for heating or cooling but the equipment doesn't respond properly, book an HVAC technician. For electrical damage or a breaker problem, call a licensed electrician.

Have the full model number ready, plus what the display shows, the room temperature, your target and the System and Fan settings. Say what happened after the five-minute wait and whether the problem started after an installation or a power outage. That helps them tell a thermostat problem from an equipment problem without guessing at parts.