You set the thermostat to cool, “Cool On” starts flashing, and nothing happens. Most of the time you only need to wait. The flashing is a built-in delay. Sometimes, though, it keeps blinking and the system never starts. Here is how to tell which situation you are in.
Why it blinks: compressor protection
Many thermostats, including the common Honeywell models, flash “Cool On” or “Heat On” while they hold off starting the equipment. Honeywell Home describes this as a delay mode that can last up to five minutes and protects the equipment from short cycling.
The reason is the compressor. Right after it shuts off, the refrigerant pressures inside the system are still unbalanced. Restarting against that pressure is hard on the compressor and can trip it. So after the system turns off, or after a power interruption, or after you change a setting, the thermostat waits before calling for cooling again. When the delay ends, the message stops flashing and turns solid.
When it is normal
- Right after you change the temperature or switch from off to cool
- Right after the power comes back from an outage or a tripped breaker
- Right after new batteries go in
- Right after the system has just finished a cycle
In all of these, wait five minutes. If the message goes solid and the system starts, nothing is wrong.
When it keeps blinking
If it flashes for much longer than five minutes, or the message goes solid but nothing comes on, something else is going on. Honeywell Home notes that a delay lasting longer than five minutes can point to a voltage problem at the thermostat. The usual causes:
- Weak batteries. Some models flash or behave oddly when batteries are low. Replace them first.
- No power from the system. A tripped breaker, an air handler door safety switch not engaged, or a blown 3 amp fuse on the control board can all cut power to the thermostat circuit. Our article on a furnace or air handler blowing its fuse covers that one.
- A condensate float switch that tripped. In cooling season, a clogged drain lifts a float switch and cuts the signal. The thermostat keeps calling and nothing happens.
- A loose or broken thermostat wire. Especially after a thermostat swap.
- A thermostat that has failed. Less common than the others.
If “Cool On” is solid but the AC is not running
The thermostat has done its job and is calling for cooling. The problem is downstream. Go outside and check whether the outdoor unit is running.
- Outdoor unit silent. Check the outdoor breaker and the disconnect box. Then it is likely a capacitor, a contactor or wiring. See why an AC will not turn on.
- Outdoor unit running, indoor fan not. The air handler side: blower motor, board or a door switch.
- Everything running but the air is warm. A different problem, covered in AC blowing warm air.
Heat On blinking works the same way
On heat pumps the same delay applies in heating mode, because the same compressor runs both. On a gas furnace, “Heat On” flashing is usually a brief delay while the furnace runs its startup sequence. If it goes solid and the furnace never lights, see furnace will not turn on.
What not to do
Do not keep changing the setpoint or switching the system off and on to “reset” it. Each change restarts the delay, and repeated power cycling does exactly what the delay exists to prevent.
What we do
If the thermostat keeps blinking, we check voltage at the thermostat and at the control board, the float switch, the fuse and the wiring between them. The fix is usually a wire, a fuse or a clogged drain. Our diagnostic is a flat $69 and it is credited toward the repair. More in thermostat not working.
Frequently asked questions
What does it mean when my thermostat is blinking Cool On?
The thermostat is in a delay mode before starting the air conditioner. Honeywell Home says this delay can last up to five minutes and protects the equipment from short cycling.
How long should Cool On blink?
Up to about five minutes. If it keeps flashing much longer, the thermostat may not be getting proper voltage from the system, and something like a tripped breaker, blown fuse or float switch may be the cause.
Why does Cool On blink after I change the batteries?
Changing batteries resets the thermostat, and it starts the compressor protection delay again. Wait five minutes.
My thermostat says Cool On but the AC is not running. Why?
The thermostat is calling for cooling, so check the equipment. A silent outdoor unit points to its breaker, disconnect, capacitor or contactor. A silent indoor fan points to the air handler.
Can I bypass the thermostat delay?
You should not. The delay protects the compressor from starting against unbalanced pressures, which is hard on the most expensive part of the system.
Does Heat On blinking mean the same thing?
On a heat pump, yes, the same compressor delay applies. On a gas furnace it usually means the furnace is running its normal startup sequence.