Thermostat Showing a Red Light? Here are 7 Steps to Solve It

Uh, oh!

Is this your situation? Or at least close to it? It’s a very hot day, meaning you really need your air conditioner to be working, but there is a red light showing on your thermostat. Perhaps it’s blinking or perhaps it’s steady.

If this is what’s happening, I’ll walk you through a variety of reasons why and try to either provide you with a simple solution or tell you you need a skilled technician.

Your thermostat blinks or stays a steady red when your compressor—the outside unit of your AC—has shut down for some reason. When this happens, a signal is sent to your thermostat. However, your indoor unit doesn’t get a message and continues to run, not aware there is a problem.

So what are some of the problems that could shut your compressor down and cause it to send a message to your thermostat? And how do you get rid of that red light?

Keep reading, and I’ll tell you.

The Causes Behind Your Thermostat Showing a Red Light

As stated above, the most likely reason for a red light is that your compressor has shut itself off. You may not even be aware of it because you can still hear the inside part of the system running. However, eventually, you’ll notice that your home is getting warmer.

    Your compressor may stop running for one of these reasons
Loss of power
Failing or dead capacitor
Burned out condenser fan motor
Broken compressor contactor
Power surges and short circuits
Pressure issues
High or low discharge temperature

If your compressor isn’t running, don’t worry, it isn’t necessarily a catastrophe.

1. Loss of Power

Check your circuit breaker or fuses to see if something has tripped or blown. If this is the case, it’s a simple fix to get your compressor running again.

circuit breaker reset
Reset your circuit breaker after a loss of power

If your compressor also has a reset button, you may want to try that as well.

That’s the simple, easy-to-fix issue. Other problems are more complex and will most likely require a pro. Here’s a list of possible issues:

2. Failing or Dead Capacitor

The capacitor is something like a battery. Depending on the type of capacitor you have, it has a store of energy that the fan in your compressor uses to start and run.

As the capacitor begins to fail, and eventually die, there’s not enough juice to run either the condenser fan or the condenser itself. At this point, your unit will probably just shut down.

If this happens, you’ll need to get your capacitor replaced as soon as possible. You don’t want to leave the situation for too long because a bad capacitor can lead to other problems.

For example, a weak capacitor could cause the condenser fan motor to work harder. If this goes on for too long the fan motor could burn out, so you could also damage your compressor.

Check out this video for more info!

3. Burned Out Condenser Fan Motor

As mentioned above, some situations could cause your condenser fan to burn out. One of them is the capacitor, but another is a lack of maintenance. That or wear and tear as your unit ages can lead to a burned-out motor.

4. Broken Compressor Contactor

The contactor is a heavy-duty switch that is found on the compressor unit outside. It turns the unit on and off and powers the components of your compressor.

If the contactor is broken it could result in both your fan and condenser not running or continuing to run at the same time.

Contactors rarely fail, but they can get dirty. Bugs and debris can clog it up and stop it from working properly, but they can be cleaned and replaced fairly cheaply if necessary. You can follow the steps in the following video but if you are not comfortable with this, please call a professional.

If you want to get any replacement part – or see how much one would cost – click to enter your model number in the search bar below. Our partners at AppliancePartsPros stock almost every part with free guides on how to install them.

5. Power Surges and Short Circuits

An electrical issue anywhere in your system can cause your air conditioner to stop running. This could be because of a power surge or even old wiring.

Checking thermostat
You might want to call out a professional to check the wiring for you

You may be able to visually inspect some of your wiring, but you might want to call a pro for this.

6. Pressure Issues

Air conditioners have a high-pressure switch that will turn off the compressor when pressure is either too high or too low in the system. There are a number of things that can cause the pressure switch to engage.

  • Dirty condenser coil
  • Slow or failing fan motor
  • Fan blades going in the wrong direction
  • Too much refrigerant
  • Blocked refrigerant lines
  • Extremely high outdoor temperatures

Remember that anything that causes your compressor to stop working can lead to a red light on your thermostat.

If you have pressure issues you must turn your system off and call a technician.

7. High or Low Discharge Temperature

Your thermostat will respond to a high discharge temperature which happens when the pistons, rings, and cylinders begin to wear out and eventually heat up in the compressor. Things become so hot that the oil in the system loses its ability to lubricate.

Eventually, your compressor will shut down, and your thermostat will notify you with a red light.

Again, this needs to be addressed by a technician.

Low discharge pressure can be the result of low airflow, whether that’s from an issue with the belt or dirty filters. Your system is likely in need of serious maintenance, so a service call is probably the best idea.

What The Red Lights On Your Thermostat Mean

Each manufacturer may have its own error codes, and many of them don’t include red lights. It’s always a good idea for you to check your manual. Below are the codes for a York Stellar system.

Error Codes Problem Action
1 red flashGas valve issueCall a technician
2 red flashesHigh pressure
3 red flashesHigh discharge temperature
4 red flashesLow discharge temperature
5 red flashesDefrost issues
6 red flashesFurnace locked out
7 red flashesFaulty ambient sensor
8 red flashesBad liquid line sensor

If you have tried all of these solutions above and are still stuck, we recommend calling a local HVAC professional. Make sure to double check each nearby company for good reviews to make sure you can trust them!


As you can see from above, there are multiple reasons why your thermostat may begin flashing red

To recap here are the most common issues:

  • Loss of power
  • Failing or dead capacitor
  • Burned out condenser fan motor
  • Broken compressor contactor
  • Power surges and short circuits
  • Pressure issues
  • High or low discharge temperature

Unfortunately, most of these will require a technician to come and diagnose and fix your air conditioner or furnace. In cases where you can look after the issue yourself, I have notated that above.

Hopefully, your issue was addressed here and we were able to answer your question.

While you’re here why not check out our related posts and guides below. Maybe we can help you with something else as well.

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