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?

It turns out that blinking thermostats are fairly common. Thermostats tend to blink because it’s short cycling, it’s batteries are low, your high limit switch is on, or the thermostat has an issue connecting to your heating system.

But that’s just the short answer. 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.

female checking thermostat
Thermostat keeps blinking?

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.

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.

If you ignore it, you’re likely looking at spending a whole lot of money very soon to either repair or replace your furnace or air conditioner.

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.

Reasons for Compressor Failure

Dirty condenser coilsDirt and debris can clog the coils, the compressor can overheat and shut downInsufficient refrigerantIf you have a Freon leak, the system can overheat and malfunctionElectrical issuesThe circuit board and wiring in the compressor can oxidize and cause a power failureDamaged suction linesRefrigerant lines will age and deteriorate, preventing the flow of refrigerant, and eventually leading to a breakdownInadequate lubricationLike any motor, the compressor motor requires lubrication to keep all its parts moving as they should. If not, it can seize

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.

outside unit of air conditioner
Check the compressor for any dirt or debris.

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 red light to blink on your thermostat.

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 flame sensor rod
Reduced airflow over the heat exchanger
Defective switch
Over-sized furnace

Solutions: If the issue is your flame sensor, it’s fairly cheap to 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.

4.  Your Low Limit Switch is On

Just like the above, you also have a low-pressure or limit switch on your HVAC.

A low-pressure switch monitors your system for a pressure drop. This would often happen because of a refrigerant leak. If your air conditioner’s compressor pumps out coolant without the right amount of pressure, this can cause significant damage.

Your low pressure or low limit switch is a safety feature that will shut your system down to stop that from happening.

Solution: Check for a clogged filter, a dirty evaporator coil, or blower wheel.

If all of those check out fine, you may have insufficient coolant in your air conditioner or some other restriction. In this case, call a pro, as it’s unsafe for homeowners to deal with refrigerant.

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.

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.

Thanks for reading. Why not check our related posts below? Maybe we can help you with something else.