Thermostat says "Cool On" but the AC is not running
A steady "Cool On" means the thermostat is asking for cooling, not that the air conditioner has started. Sensi notes a thermostat cannot tell whether it really runs. Honeywell says to set Cool, put the setpoint below the room temperature and wait 5 minutes. If nothing cools after that, the problem is past the thermostat and needs a technician.
What does a steady "Cool On" mean?
It means the thermostat has decided the room needs cooling and is sending that request to your air conditioner. It does not mean the air conditioner has answered. Sensi says this plainly on its loss of cooling page: the thermostat does not know whether the cooling system is on. It tries to turn the cooling on, and if there is a problem with the system, nothing happens.
A flashing "Cool On" is the other version of the message. On the Honeywell Home T4 Pro and T6 Pro, the display flashes "Cool On" while the compressor waits a few minutes before restarting, to avoid damage. That wait is normal and is covered in thermostat blinking "Cool On". This page is about the steady message, when the wait should be over and still nothing cools.
How long should you wait before deciding it is not working?
About 5 minutes after your last change. Honeywell's T6 Pro installation guide tells you to press the Mode button until Cool is set, make sure the temperature you want is lower than the inside temperature, and wait 5 minutes for the system to respond.
That is in line with the compressor protection delays other brands publish. Carrier describes it as a built-in delay, typically 3 to 5 minutes, that protects the compressor from short-cycling. Lux P521Uc and TX500Uc thermostats wait 5 minutes by default, and the delay can be reduced to 2. Trane adds that some thermostats also delay startup after a power loss and may show "Wait" during it.
Switching the thermostat off and on does not help. Carrier says turning it off during the delay is safe, but the timer starts again when you turn it back on.
Is the thermostat really calling for cooling?
Before you blame the air conditioner, confirm the thermostat is asking for it. Other brands show the request in different ways:
| Brand | Calling for cooling | Waiting or not calling |
|---|---|---|
| Honeywell Home T4 Pro and T6 Pro | Steady "Cool On" | "Cool On" flashing during the compressor wait |
| ecobee | Blue ring around the edge, or a blue snowflake at the top | No ring and a white snowflake |
| Google Nest | The screen turns blue | Black screen: off or Eco |
| Sensi | A soft click and "Cooling" within a few seconds; Touch and Touch 2 screens turn blue | Delayed 5 minutes when "A/C Protection" is on |
| Amazon Smart Thermostat | Not stated | Not stated |
Google says that when a Nest screen changes color, the thermostat is trying to turn your system on. Amazon's check for its Smart Thermostat is to set Cool mode in the Alexa app, set the temperature lower than the reading on the display, and see whether the outdoor unit turns on.
What on the thermostat can stop the AC from starting?
ecobee lists several reasons its thermostat might not start a cooling cycle even with the mode set to Cool:
- The room is not warm enough yet. The thermostat waits for the room to pass the setpoint by a set amount, which ecobee calls the Cool Differential. See temperature differential.
- The last cycle ended recently. ecobee will not start cooling until at least 5 minutes after the last cycle ended.
- A time-of-use setting. ecobee lists Time of Use among the reasons it may hold off.
- It is too cold outside. ecobee will not turn the equipment on when the outdoor temperature is below its Compressor Min Outdoor Temperature setting.
The outdoor limit is easy to miss on a mild day. ecobee says it is not recommended to run an air conditioner below 60°F outside, and Honeywell's T4 Pro product information says not to run cooling below 50°F (10°C), to prevent possible compressor damage. ecobee's threshold settings, including Compressor Min Outdoor Temperature, are set only on the thermostat itself, and ecobee does not list their defaults.
| Brand | Lowest outdoor temperature for cooling(°F) | How it applies |
|---|---|---|
| Honeywell Home T4 Pro | 50 | Product information says not to run cooling below this |
| ecobee | 60 | Recommendation in ecobee cooling FAQ |
| ecobee (setting) | Not stated | "Compressor Min Outdoor Temperature" blocks cooling below the value set |
On Sensi Lite and Smart thermostats, Sensi adds that low batteries may stop the thermostat from turning on your system.
Is it just the fan?
Check the fan setting. The Sensi Touch 2 manual says that with the fan set to "On" it may sound as if the system is cooling when only the fan is running, and tells you to set the fan to "Auto". Air moving from the vents does not prove the air conditioner started. The two settings are compared in fan "On" vs "Auto", and the in-between option is explained under fan circulate mode.
What should you check now?
Is the mode Cool, with the setpoint below the room temperature?
- Yes
Has it been at least 5 minutes since you last changed anything?
- Yes
Is cold air coming from a vent with the fan set to "Auto"?
- Yes
The air conditioner is running. Give the room time to cool.
- No
Replace the thermostat batteries if it uses them. If there is still no cooling, the problem is past the thermostat. Check the signs at the bottom of this page.
- Yes
- No
Leave it alone until 5 minutes have passed. A compressor protection delay may still be running.
- Yes
- No
Set the mode to Cool and the setpoint below the room temperature, then wait 5 minutes. The T6 Pro accepts cool setpoints from 50°F to 99°F.
Sensi's own advice when its loss of cooling alert appears is the same final check: hold a hand to a vent and feel whether the air is cold. If the thermostat is calling, the delay is over and the air is not cold, a licensed HVAC technician needs to look at the system. For other Honeywell Home messages, see the Honeywell Home hub.
Frequently asked questions
- Is a steady "Cool On" different from a blinking one?
- Yes. On Honeywell Home T4 Pro and T6 Pro thermostats, a flashing "Cool On" means the compressor protection wait is running, so the delay is expected. A steady "Cool On" means that wait is over and the thermostat is calling for cooling now.
- Why does the air from my vents feel warm when the thermostat says "Cool On"?
- If the fan is set to "On", it runs whether or not the cooling does, so moving air alone does not prove the air conditioner started. If the air stays warm with the fan on "Auto" and the delay is over, the problem is past the thermostat.
- Will turning the thermostat off and on again make the AC start?
- No. Carrier says it is safe to turn the thermostat off during the delay, but the delay timer starts over when you turn it back on. Set the temperature once and wait.
- Can my AC refuse to run because it is cold outside?
- Yes, on some systems. The Honeywell Home T4 Pro product information says not to run cooling when it is below 50°F outside, and ecobee thermostats will not start the equipment below their Compressor Min Outdoor Temperature setting.