AC Thermostat Not Turning On — What To Do?

Having your thermostat malfunction during the middle of summer can be the biggest inconvenience. There are a few reasons that may cause the thermostat to fail to power on, though it may take a bit of troubleshooting to determine the specific reason for the issue. We have researched the most common reasons why this may happen and what to do when it does.

Here are a few reasons why your AC thermostat may not turn on:

  • The batteries have died
  • The thermostat unit has broken
  • The circuit breaker has tripped
  • The safety switch has tripped
  • The AC evaporator coils are dirty
  • The air filter is dirty

Even if your air conditioner system is brand new, you can experience thermostat issues very early on. It's best to keep your instructional manual handy for this very reason. Continue reading to learn how to troubleshoot the most common thermostat issues and how to determine if it's time to call a pro for the job.

Caucasian female hand pressing button on a modern electronic thermostat timer on wall of a modern home with focus on the screen and fingers of the woman, AC Thermostat Not Turning On — What To Do?

Most Common Reasons Why AC Thermostat Won't Turn On

The batteries have died

If the thermostat is no longer malfunctioning, chances are that the batteries are dead. This is probably one of the most common reasons why the thermostat will fail to power on—and the easiest one to resolve.

Even if you believe that you've recently replaced the batteries, try replacing them with a new pair to see if it solves the issue.

The thermostat unit has broken

Your average thermostat will last about ten years or more, depending on its making model. Over the years, you may notice the thermostat beginning to malfunction, which is a clear sign that it may be time to replace it.

And sometimes, the thermostat may have wiring issues, bad batteries, or may need to be cleaned, so be sure to do a bit of troubleshooting before reaching out to a technician to have it replaced.

The circuit breaker has tripped

A faulty thermostat can easily trip your home's circuit breaker. If you find that the powering on of the air conditioner repeatedly trips the breaker, chances are that there may be an issue with the wiring of the thermostat.

To reset the breaker, simply flip the switch back on. If it doesn't fix the issue or if you find that it's happening more often, you may need to reach out to an HVAC professional.

The safety switch has tripped

Sometimes a simple underlying issue (such as a short in the electrical wiring) with your air conditioner system can cause the transformer to stop functioning correctly.

When this happens, it will no longer send voltage to the thermostat, causing it to power off or malfunction. You can check the sensor on the safety switch (which is located in the drain pan). If the sensor goes off, you'll likely need to call a technician to inspect the unit.

The AC evaporator coils are dirty

Sometimes the thermostat may not be the issue at all. The evaporator coils located in the air conditioner unit outdoors can become dirty and, in turn, will restrict airflow. Over time, the system won't be able to exchange heat within the home efficiently, and it may power off as a result.

Checking the evaporator coils on your condensing unit is an important step to perform before reaching out to an HVAC service pro. If the coils are dirty, you clean them using a coil cleaner and water hose with a spray attachment.

Check out this coil cleaner on Amazon.

The air filter is dirty

Surprisingly, a dirty air filter can also cause your thermostat to power off. A clogged filter will restrict the airflow coming from a blower motor, and over time it will cause the condenser to work harder than it usually would. Eventually, the condenser may fail as a result.

If the condenser fails, the air conditioning unit will not power on at all. In this case, you'll need to have an HVAC technician replace the condenser unit. This is why it's best to replace your air filters at least once a month.

Why would an AC suddenly stop working?

A hand using remote control open the air conditioner, AC Thermostat Keeps Shutting Off — What To Do?

If your AC suddenly stops working, chances are it's an issue with a major component, electrical problems, or restricted airflow somewhere in the system. Let's take a look at the most common reasons why this may occur.

The capacitor is dead

If your air conditioning unit is over 10 to 15 years old, it could be that the capacitor has died. Though capacitors can last up to 20 years, they can go out as early as 7-10 years. If you notice that the air conditioner has trouble powering on and off and then stops abruptly, chances are that it's the capacitor.

You may also hear clicking noises when the air conditioner stops; this is usually a sign that the capacitor is starting to malfunction. If the capacitor is dead, you'll need to reach out to an HVAC technician to replace it.

The fan is broken

The blower fan on an air conditioning unit can also become faulty. If the air conditioner kicks on, but you don't hear any air coming through the vents, the blower fan may be the culprit. To troubleshoot the issue, look at the outside condenser unit when the unit is turned on to see if the blower fan is turning. If it's not, it could need a replacement, cleaning, or fuse replacement.

The condenser is dirty

A dirty condenser can also cause the AC to suddenly stop working. The condenser's job is to release the heat collected from the outside environment into the home as clean, cold air.

However, if the unit's coils and fins are restricted due to dirt and debris, it will begin to work harder to disperse heat. As a result, it will draw a stronger current than necessary to perform. Ultimately, the unit can cause the system's circuit breaker to trip, resulting in the system not powering on at all.

The circuit breaker is tripped

When the circuit breaker for the AC system is tripped, the unit will no longer power on. Unfortunately, there are quite a few components and scenarios that may cause this to occur.

If the issue is due to bad wiring, the high voltage usage will cause the breaker to turn off every time the unit powers on. And if this is the case, a technician will need to look at the wires to see what the underlying problem may be.

The AC unit has run its course

At the 10-year mark, you may begin to notice malfunctions with your AC system. And here are a few signs they indicate that the unit may require replacement:

  • You have completed over two or three repairs in one summer season
  • Your energy bills are suddenly increasing, but your usage remains the same
  • The air conditioner system suddenly sounds a lot louder or has begun making weird noises, including rattling, screeching, or loud banging
  • The air conditioner keeps experiencing refrigerant leak issues, causing you to have to get a recharge once or twice a year

What is wrong when your thermostat goes blank?

Caucasian female hand pressing button on a modern electronic thermostat timer on wall of a modern home with focus on the screen and fingers of the woman, AC Thermostat Not Turning On — What To Do?

If your thermostat goes blank, more likely than not, the batteries need to be replaced. If you replace the batteries and the thermostat still doesn't power on, it's likely that the circuit breaker has been tripped or the thermostat is malfunctioning. If it's the latter, the thermostat may need cleaning or replacement.

How do I know if my thermostat fuse is blown?

You can check the thermostat fuse by locating it at your circuit breaker. You'll need to use a multimeter to test the fuse. The reading on the multimeter will let you know the fuse has continuity or if it is dead.

Find this multimeter on Amazon.

Where is the fuse for a thermostat?

The fuse for the thermostat is located at your circuit breaker box. It will be located on the same circuit as the furnace, and AC is on.

How do you turn your AC on without a thermostat?

There is a way to bypass the thermostat on your air conditioning unit. However, it isn't recommended if you don't have experience with electrical components. It usually requires jump-starting the wires by twisting them by hand. However, if you have no electrical license or experience, it's best to hire an electrician to fix the thermostat issue for you.

Is it hard to replace a thermostat?

Setting thermostat to 78 degree, AC Thermostat Keeps Shutting Off — What To Do?

It's not physically grueling work, but it does require knowledge and experience with electrical components. Replacing a thermostat requires electrical and HVAC expertise and skills. Doing this on your own is not recommended, as it can cause personal injury and serious issues to your home's electrical and HVAC systems.

Wrapping Things Up

We hope that this post has helped illustrate how to troubleshoot a thermostat that doesn't power on. Remember, electrical issues are very serious, and it's always best to leave this type of work to the pros to be on the safe side.

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