Why Your Thermostat Keeps Blinking – And How to Fix It

For much of the year, most of us depend on a furnace or air conditioner to heat or cool our homes. But what if your thermostat keeps blinking without raising or lowering the temperature?

Here’s our HVAC expert’s explanation:

Thermostats normally blink the selected mode word (Heat, Cool) in a programmed wait time. 

This is built into the program and it is normally changeable. Most digital thermostats are set by default to wait 5 minutes in between cooling calls which is a way of protecting the compressor from starting before all the pressures have equalized. In the heating mode (with gas heat) it is normal set by default for 2 minutes. 

This is the main reason you may observe a blinking of Heat or Cool as evidence for a possible short cycling issue. 

This should only be a cause for concern if the system is continuing to go through a cycling of on and off with a lot of blinks in between, rather than just pausing for 5 minutes before operating normally.

That’s the short answer, anyway. Keep reading, and I’ll provide you with the list of reasons why your thermostat is blinking. And tell you what you need to do to fix the issue (though this may involve calling a pro).

Your Thermostat Keeps Blinking – What Does it Mean?

If only a thermostat could talk to us and tell us exactly what the problem is without making us play a guessing game.

I have no doubt that future is nearly here—if it isn’t already, and I missed the news release—but for now, we need to rely on the obscure messaging of our thermostats.

We need to translate all that flashing and blinking.

Typically, the answer is because our system is short cycling.

What is Short Cycling?

Since we can usually hear the woosh of air through the vents when the HVAC kicks in, most of us are aware of the fact that the furnace and air conditioner cycle on and off.

Depending on how cold or hot it is outside on any given day, this translates into how often they do so. The more extreme the temperature is outside, the harder our HVAC appliances need to work. In this case, they may cycle every 12 to 15 minutes. At other times this period may extend to 20 minutes.

This is normal behavior.

Short cycling is when something is causing our HVAC to cycle faster than it should. The makes the appliances work too hard and has the potential to damage them.

If your system is short cycling, there is a failsafe built-in. And your thermostat flashes to communicate that, telling you that it’s temporarily shut your system down. This is called Delay Mode.

Essentially, the blinks are a call for heating or cooling, but that the system is waiting a period before coming back on. This is adjustable, but as default it’s usually 5 minutes for cooling and 2 minutes for heating.

When this happens, you can typically expect it to restart in about 5 minutes. But note that until you fix whatever is causing the short cycle, this process will just keep repeating itself.

This is a fairly normal occurrence – but if it keeps on happening, then you might have a larger underlying issue. 

What Causes Short Cycling?

Several things can cause your system to short cycle.

Possible Causes of Short Cycling
Incorrect SizingYour furnace or air conditioner is too big for your space
You May Need Repair WorkThere could be issues with electrical components, the condensate switch, the flame sensor, or something blocking airflow.
Maintenance IssuesKeep up with regular maintenance like changing air filters and inspections.
Problems with the Thermostat ItselfYou may need to replace your thermostat. Or you may have it placed somewhere too close to a hot or cold source.

If your thermostat is blinking and telling you it’s in delay mode, it could be because it detected one of the issues mentioned above and has temporarily shut your system down.

But you still need to find and address whatever the issue is that caused it because until you do, the blinking will just keep happening.

Other Reasons Why Your Thermostat Might be Blinking

Short cycling and the following delay mode is only one reason for a blinking thermostat, but it is the most common.

If you’ve ruled that out, here are other possible reasons.

1. Your Compressor Has Shut Off

Your compressor is the part of your air conditioner that sits outside beside your house.

There are several reasons why a compressor could fail and shut down. A few could be:

  • Dirty condenser coils. Dirt and debris can clog the coils, the compressor can overheat and shut down
  • Insufficient refrigerant. If you have a Freon leak, the system can overheat and malfunction
  • Electrical issues. The circuit board and wiring in the compressor can oxidize and cause a power failure
  • Damaged suction lines. Refrigerant lines will age and deteriorate, preventing the flow of refrigerant, and eventually leading to a breakdown
  • Inadequate lubrication. Like any motor, the compressor motor requires lubrication to keep all its parts moving as they should.

Solution: If your condenser coils are dirty, you can clean them yourself. If your problem is any of the other reasons for a compressor to fail, you need to call a pro to come and look at it.

Air Conditioner Hissing
Cleaning your condenser coils is an easy task you can perform yourself.

2. Dead or Weak Batteries

If the voltage in your batteries has dropped and they’re weak, your thermostat may be blinking as a warning. Not all thermostats have this feature, so check your manual to see if yours does. If not, your problem could be something else.

Solution: If you have a battery tester, take out your batteries and test them. Replace if necessary. If not, replace them anyway and see if that stops your thermostat from blinking.

3.  Your High Limit Switch is On

The high limit switch on your furnace or air conditioner, also called the temperature switch, high-temperature switch, or just the limit switch. This is a part of the safety system of every system.

If your limit switch has tripped, it can cause a light to blink – but NOT on your thermostat! This blink will typically appear on the furnace circuit board, and typically isn’t communicated to the thermostat (unless you have a really high end model).

The function of the switch is to shut off the burners and turn on the blower fan if the furnace reaches unsafe temps. The burners will come back on when temperatures return to a normal range.

Depending on the furnace, if this switch trips too many times in a row, the furnace can go into lockdown—who else has come to hate that word?—mode.

Reasons for a Limit Switch to Trip
Dirty filter
Reduced airflow over the heat exchanger
Defective switch
Over-sized furnace

Solutions: If the issue is your flame sensor, it’s fairly cheap to clean or replace. This YouTube video provides a troubleshooting and replacement guide.

If you have an airflow issue, you should check your filters and replace as necessary.

Over time, your limit switch can become faulty, but you can repair or replace it yourself.

Have a Heat Pump?

For those of you who have a heat pump instead of an air conditioner and or furnace, you can still be surprised by a blinking thermostat.

If this is your situation, check to see if you have an ice buildup anywhere. It could be that the defrost cycle is malfunctioning and causing your thermostat to blink.

Heat pumps have the added feature of a defrost that allows the outdoor unit to turn on the electric heat strips when it goes in the defrost cycle.

It’s fairly common for the thermostat to light a led bulb or display AUX Heat in this case. This mode could also blink. This just tells you that your unit is in defrost.

It might be common to see this often on a cold day, but if you see it often on a mild winter day, you could have a problem which usually centers around refrigerant pressures.

Solution:  One thing to check is your filters. Dirty filters can cause an ice buildup. Also, try clearing away any debris or obstructions around the outdoor unit.


If your thermostat is blinking, you could have some issues that require a fix that you’re not comfortable with.

If that’s the case, don’t attempt fixing anything yourself, even though I have provided tutorials.

Having said that, some of the solutions require simple things like changing filters or batteries.

Here’s a quick recap of the issues that might cause a blinking thermostat.

  • Your system is short cycling
  • Your compressor has shut down
  • Your batteries are dead or weak
  • Your high limit switch has tripped

Hopefully, you were able to find an answer and solution to your situation.

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I've been helping homeowners with appliance repair since 2016. Starting out as an enthusiastic amateur, I've since worked with many Appliance, HVAC, and DIY experts over the last 7+ years. My mission is to help fix your appliances and prevent future issues - saving you stress, time, and money. Visit my author page to learn more! Read more