The Meaning of Thermostat Showing Red Light Flashing or Stays On

The thermostat allows you to control the temperature changes in the house. In cars, it helps in regulating the heating and cooling engine process. The thermostat comes with a red light that senses the occurrence of a certain problem. As a result, multiple codes are sent one after another, notifying the owner of issues such as blown-out thermostat sensors.

Many thermostats’ red light flashes or stays on when the outdoor system unit is “locked out”. If your thermostat red light keeps flashing or fails to turn on, consider troubleshooting the problem immediately to regulate thermostat damages. Therefore, it is important to troubleshoot the thermostat to increase its performance. The information below gives the meaning of red light flashing or staying on and the possible causes.

Meaning of thermostat red light flashing or stays on

thermostat red light flashing or stays on meaning

Experiencing a thermostat red light flashing or staying on is a common problem, especially when driving a car. A red light signifies you to stop the car during an emergency. When a thermostat flashes red light, it indicates “locking out” of the outdoor unit. In most cases, the outdoors shuts itself because of a problem and signifies an issue.

Once the outdoor “locks out,” the indoor unit continues functioning because it does not sense any occurrence of a problem. However, the thermostat does not normally function as usual despite the indoor unit functioning even if it keeps running because it requires both the indoor and outdoor units.

When the thermostat red light flashes light, a code is sent continuously several times, approximately three to ten times. The process repeats itself, resulting in the red light flashlight staying on. To solve the issue of red light flashing, first, reboot the thermostat by switching it off. Let the thermostat sit for a couple of seconds, approximately thirty to forty seconds. Then, switch on the thermostat to eliminate the problem.

Different thermostats use different “shutting off” techniques during the rebooting process. For example, some thermostats require switching off power to the indoor unit, while others require thermostat settings. If you reboot the thermostat and persist when turning it on, contact a skilled technician for better services.

Some thermostats “lock out” due to climate changes, and troubleshooting such a problem requires you to reboot the whole system to get accurate results. If the problem reoccurs, consider seeking help from the service center. Also, follow the manufacturer’s instructions on the rebooting process to avoid flashing all important information.

Causes of thermostat red light flashing or staying on

Causes of thermostat red light flashing or staying on

The following list provides possible causes of red light flashing or staying on, and the problems are fixable whether by an individual or service center.

  • Changes in weather conditions

During cold seasons, especially winter, the thermostat may defrost too much, failing to function as usual. Thermostat freezing, usually resulting from snow and ice, triggers the flashlight’s normal functioning, resulting in the thermostat red light flashing or failing to turn off.

  • Low temperature and pressure while the switch is on.

In most cases, low temperatures and pressure result from the clogged thermostat air filter, damaged fan motor, thermostat overcharging, among others, when turning on the thermostat switch. Once the thermostat heating system fails to function, a code is sent indicating occurrence or a problem. The code sent delivers information by failing to shut off, or the red light continuously flashes.

  • Low thermostat temperature discharge

Low-temperature discharge is a result of a failing thermostat. Failing the thermostat leads to the production of red light that continuously flashes, locking out the outdoor unit as a sign of a serious problem.

  • Damaged thermostat sensor

Sometimes, the thermostat flashlight senses a failing thermostat sensor and sends multiple codes continuously, resulting in a flashing red light. If the thermostat keeps flashing the red light, consider checking the condition of the thermostat sensor and try fixing it by following the recommended steps. If the problem persists, consider replacing the thermostat sensor or contacting a skilled technician.

  • Damaged outdoor unit

If the indoor thermostat unit is functioning correctly, but the red light keeps flashing, the outdoor unit might have some problems. If the red light keeps flashing, it is a sign of the outdoor unit “locking out,” meaning the outdoor unit is not functioning at all or as required.

If you experience outdoor “locking out”, consider contacting a skilled HVAC technician to help you fix the outdoor unit system. Ignoring the damaged outdoor unit system increases electricity bills since the indoor unit keeps running without satisfying the thermostat functioning.

  • Low battery

If the red light keeps flashing, it may mean the battery is low. Low battery requires quick charging or replacement if completely damaged. Replacing the battery with a newly charged battery is the best solution to solving the low battery problem.

To replace the battery, first, disconnect the thermostat from the wall using appropriate tools. Next, open the thermostat by unscrewing all bolts using a screwdriver. Next, remove the old battery from the thermostat and replace a new thermostat battery. Finally, close the thermostat and test it. If the red light continues to flash, contact an HVAC technician to help you solve the red light flashing problem.

Steps how to troubleshoot thermostat red light flashing or staying on

Steps how to troubleshoot thermostat red light flashing or staying on

The following steps are necessary when fixing the thermostat red light problem.

i. Find the total number of flashes by counting them. Then, refer to the manufacturer’s manual book on the steps of troubleshooting the red light depending on the number of flashes.

ii.  Switch off the thermostat and let it sit for a couple of seconds, approximately 35 seconds. Then, turn the thermostat on to eliminate red light flashing. Turning off and on the thermostat is a way of rebooting it, and in some thermostats, the rebooting process may take up to fifteen seconds.

iii. Switch off the indoor unit power and let it sit for around eight minutes. Power the indoor unit on again to troubleshoot the red light flashing. Repeat the process until the red light turns off.

iv. Switch off the circuit breakers in the outdoor unit if the issue results from the outdoor unit. Leave the circuit breaker for around 10 minutes and connect it back to troubleshoot the problem.


 Thermostat red light flashing or staying on brings discomfort to the owner. Luckily, the article above shows the steps to follow when troubleshooting the problem. The steps are easy to follow for both professionals and non-professionals.

Also, the information above describes the possible causes of red light flashing. To obtain positive results, ensure you follow the steps above carefully. Finally, remember to follow the manufacturer’s instructions when replacing the thermostat or the components of the thermostat.

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