Thermostat Display Messages Explained: What Your Screen Is Telling You

1. Thermostat messages at a glance

Thermostats have very little room on the screen, so a few words or an icon have to tell you a lot. Most messages fall into four groups: normal waiting (delays and recovery), you overrode the schedule (holds), which equipment is running (heat, cool, aux, fan), and something needs attention (power, batteries, wiring, filter). Only the last group needs action.

2. “Cool On” or “Heat On” flashing, and “Wait”

This is the most searched thermostat message, and it is almost never a fault. An air conditioner or heat pump compressor should not restart until the high and low refrigerant pressures have evened out after it stops. Restarting against that pressure difference strains the compressor motor and can trip its breaker. So the thermostat enforces a minimum off time, usually up to about 5 minutes, after the compressor stops and after power returns.

What the flashing looks like on different thermostats

• Honeywell Home and many programmable models: the words “Cool On” or “Heat On” flash, then stay steady once the compressor starts.
• Other models: the word “Wait”, a blinking snowflake (cooling) or flame (heat pump heating), or a small clock icon.
• Smart thermostats: a message such as “Delayed”, “Cooling in 2 min” or an equipment-protection notice in the app.

When it is a real problem

Wait about 10 minutes. If “Cool On” is steady but the outdoor unit isn’t running, or it keeps flashing far longer than 5 minutes, the delay isn’t the cause. Work through thermostat clicking but AC not turning on, which follows the cooling call from the Y wire to the outdoor unit. Check the Y wire and the condensate float switch, which cuts the cooling call when the drain pan fills.

Don’t switch the delay off in the installer settings to make the AC start faster. It exists to protect the most expensive part of the system.

3. “In Recovery”, Smart Recovery and Early-On

If your schedule says 68°F at 6:30 AM, an older thermostat starts the furnace at 6:30 and the house is cold until 7. A thermostat with recovery learns how long your system takes to warm up or cool down and starts early, so it reaches 68°F at 6:30. While it does that, it shows a message:

• Honeywell Home: “In Recovery” (Adaptive Intelligent Recovery)
• ecobee: Smart Recovery (shown in the app and system history)
• Google Nest: Early-On

It is normal and costs less than leaving the temperature high all night. On a heat pump, recovery also matters for cost: a good thermostat warms the house slowly with the compressor instead of calling expensive aux heat for a fast recovery. See heat pump thermostat settings. If you’d rather it didn’t start early, recovery can usually be turned off in the settings menu.

4. Temporary hold, permanent hold and vacation hold

“Hold” means a temperature you chose by hand is overriding the schedule. What matters is how long the override lasts.

On smart thermostats you can usually choose what a manual change does (for example until the next period, for 2 or 4 hours, or indefinitely). If your thermostat “keeps going back” to another temperature, it is following its schedule after a temporary hold ended; if it “never changes”, it is probably on a permanent hold. For what temperatures to put in the schedule, see schedules and smart features.

5. “Aux Heat”, “Em Heat” and “Emergency Heat”

These appear only on heat pump systems. Aux Heat (or “Aux”, “Auxiliary Heat On”) means the thermostat has brought on the backup heat, usually electric heat strips in the air handler, to help the heat pump. That is expected:

• in very cold weather, when the heat pump alone can’t keep up;
• during defrost, when the outdoor unit briefly runs in cooling mode to melt frost;
• after a big jump in the setting, such as a deep overnight setback.

Em Heat or Emergency Heat is different: you selected it, and it turns the heat pump off completely and heats with backup heat only. It is meant for when the outdoor unit is broken. Because electric strip heat costs much more to run than a working heat pump, switch back to Heat once the heat pump is fixed. The full comparison is in aux heat vs emergency heat; if Aux Heat is on most of the time in mild weather, see heat pump using aux heat too much. Wiring for these is on the W2/AUX and E terminal pages.

6. Smart thermostat messages

Smart thermostats have bigger screens and apps, so they spell more out, but brands word things differently. These are the ones people search for most.

“Delayed” or “in 2 min”

The thermostat is holding off heating or cooling. Usually this is the same equipment-protection delay as a flashing “Cool On”. On thermostats that keep a small battery charged from the HVAC wires (Google Nest models, for example), it can also mean the battery is low and the thermostat is pausing so it can recharge. If you see “Delayed” often, especially with Wi-Fi dropouts or the screen going dark, the thermostat probably needs a C wire. See no C wire? your options for every brand and the C wire guide.

“Calibrating”

ecobee thermostats show “Calibrating” for a few minutes after they restart, while the sensors settle. Heating and cooling wait until it finishes. If it never finishes, or the thermostat keeps rebooting into calibrating, it is usually restarting because of a power problem: check the wiring and the C wire.

“No power to Rh wire” or “no power to R”

Covered in detail in the next section: the thermostat isn’t getting 24 V.

“Wire not detected” or “Y1 not detected”

Usually a wire not pushed fully into its connector, or a terminal that reports no voltage. See thermostat wire not detected.

“Eco”, “Away”, “Home and Away”, “Eco+”

Energy-saving modes. The thermostat believes no one is home (from its motion sensor or your phone’s location) or is following a utility or energy-saving program, so it holds a wider range, for example 62°F heating and 80°F cooling. If it switches to Eco while you are home, check the motion sensors and the phone location settings in the app.

“Offline” or a crossed-out Wi-Fi icon

The app can’t reach the thermostat. The thermostat still runs your schedule on its own. Restart the router, check the Wi-Fi password hasn’t changed, and look for power problems: a thermostat that keeps dropping Wi-Fi is often one starved of power without a C wire.

Low battery on a smart thermostat

A wire-powered smart thermostat warning about a low battery is almost always a power supply issue, not batteries. Add a C wire or an adapter such as the ecobee PEK (for ecobee). Amazon and ecobee models need a C wire or adapter anyway; Nest models can get by without one on many systems but behave like this when they can’t.

7. “No power to Rh” and a blank screen

A thermostat on a 24 V system gets its power from a transformer in the furnace or air handler, through the R (or Rh/Rc) wire. If that power stops, a smart thermostat says “No power to Rh” (or R) and may go dark; a battery thermostat may still show the temperature but nothing turns on. Check, in this order:

1. The furnace or air handler switch, usually a light switch on or near the unit, often turned off by accident.
2. The breaker for the furnace or air handler.
3. The furnace door. The lower door presses a safety switch; if it isn’t seated, the furnace has no power.
4. The float switch on the condensate drain or drain pan. A full pan cuts the 24 V circuit on many installations. Clear the drain.
5. The 3 or 5 A fuse on the furnace control board. If it is blown, look for the short (often a wire touching metal, or R and C touched together during an install) before replacing it.
6. The R wire at both ends: properly stripped, fully seated, not broken in the wall.

To confirm, measure about 24 to 28 V AC between R and C at the thermostat: see how to test thermostat wires with a multimeter. If the screen went blank right after a power cut or a battery change, see thermostat not working after a power outage.

8. Thermostat symbols and icons

Icons are more consistent between brands than words. A steady icon means that equipment is running; a flashing heat or cool icon usually means a call is waiting on a delay.

• Snowflake blinking: cooling has been called but is waiting on the compressor delay. Steady means cooling is running.
• Flame blinking: on a heat pump, the same compressor delay in heating mode. On a gas furnace it usually just means heating is on or starting.
• Fan icon spinning or shown with “On”: the fan is set to On and runs nonstop. Auto runs it only with heating or cooling. See fan Auto vs On.
• Leaf: the current setting is energy-efficient (Nest), or an energy-saving mode is active.
• Water drop: humidifying or dehumidifying is active, or the screen is showing indoor humidity.
• “Auto” mode with both a flame and a snowflake: the thermostat switches between heating and cooling by itself. It keeps the two settings a few degrees apart (often about 2 to 3°F, sometimes called the deadband) so it doesn’t bounce between them.

9. Low battery, keypad lock and filter reminders

Low battery (“Lo Batt”, “Replace Battery”, battery icon)

On a battery-powered thermostat, replace both batteries with fresh alkaline batteries of the type printed in the battery slot (usually AA or AAA). Many models give a warning for weeks before they stop working, but don’t wait: some thermostats stop calling for heat when batteries die. Settings are usually kept in memory, but check the clock and schedule afterwards. If the screen shows a low-battery warning right after new batteries, the batteries may be the wrong type or in backwards.

Keypad locked (padlock or “Locked”)

Lock is used by landlords, offices and parents. Most thermostats unlock from the menu with a PIN, or by holding a button combination for several seconds; smart thermostats unlock in the app. The steps vary by model, so look up the manual for the model number on the back of the faceplate.

“Filter”, “Change Filter”, “Replace Filter”

This is a timer, not a sensor: the thermostat counts run time or days since you last reset it. Change the filter (most 1-inch filters every 1 to 3 months; thicker media filters less often), then reset the reminder, usually by pressing and holding the Filter message or from the menu. The reminder doesn’t affect how the system runs, but a clogged filter does: it can make a furnace overheat and short-cycle, or an AC coil freeze.

Wrong time or the schedule “forgot” itself

Usually a power cut on a thermostat with dead backup batteries, or a battery change that took too long. Wi-Fi thermostats set the clock automatically once reconnected. Some older thermostats need daylight saving time switched on in the menu.

10. It says heating or cooling, but nothing happens

If the screen shows steady “Heat On” or “Cool On” (no flashing, no delay message) for more than 10 minutes and nothing runs, the thermostat is asking for heat or cooling but the request isn’t reaching the equipment, or the equipment isn’t answering. Work through it from the wall outward:

1. Check the mode and settings: Heat mode with the setting above room temperature (or Cool with it below). For a heat pump, a wrong O/B setting makes it heat when you ask for cooling: see the O/B wire.
2. Listen at the furnace or air handler. Nothing at all, not even the inducer motor? Check power as in section 7, then the blink code.
3. Cooling: does the indoor fan run but not the outdoor unit? Follow thermostat clicking but AC not turning on.
4. Just installed a new thermostat? Use the smart thermostat troubleshooting checklist and the settings to check after installing, and compare your wires with the thermostat wiring diagrams.

If the screen shows “Heat On” and the furnace runs but the air is cold, or “Cool On” with warm air, the thermostat has done its job: that is a furnace or AC problem (flame sensor, refrigerant, a frozen coil or a dirty filter) for a technician, unless the cause is simply a heat pump in defrost.

11. Error codes and furnace blink codes

Thermostat error codes (letters and numbers such as E-codes or alert numbers) are specific to each model. The same code means different things on different brands, so always look up the code in the manual or support site for your exact model number. Most fall into a few groups: a failed temperature sensor, a lost connection to the equipment or a remote sensor, a wiring or power fault, or a reminder.

Often, though, the thermostat is fine and the furnace has locked itself out. Most modern furnaces report their own faults with a blinking status light on the control board.

1. Look before you open. Read the light through the window in the lower door. Opening the door releases the door safety switch, which cuts power, and many boards then lose the code (some keep a fault history).
2. Count the flashes between the pauses. Three flashes, pause, three flashes means code 3. Some boards use long and short flashes or two colors.
3. Match it to the chart printed on the inside of the door or in the installation manual. Never use a chart from a different furnace.
4. A steady light or a slow, even blink usually means normal standby. No light at all means no power to the board (see section 7).

Common causes the codes point to are a pressure switch problem (often a blocked vent or condensate line), a dirty flame sensor, an open limit switch (often a clogged filter), ignition failure or lockout, and reversed polarity. Changing the filter and clearing the condensate drain are safe DIY fixes; gas valves, burners, igniters and anything with the gas line are for a licensed technician.

12. Frequently asked questions