Troubleshooting When Your Thermostat Won’t Turn Off the AC

It’s the middle of summer and you realize that your air conditioner seems to be running non-stop, even when you don’t want it on. Upon further inspection, you see that your thermostat is set to a normal temperature, but the AC just keeps blowing cold air. Dealing with a thermostat that won’t turn off your air conditioner can be frustrating, not to mention a drain on your energy bill. In this blog post, we’ll walk through the common reasons why your thermostat might not be shutting off the AC like it should, along with troubleshooting tips to get it working properly again.

Causes of a Thermostat That Won’t Shut Off the AC

There are a few key reasons why your thermostat may not be properly communicating with your AC system to turn it off when it reaches the set temperature:

Faulty Thermostat

The most obvious culprit is a malfunctioning thermostat. Thermostats are electronic devices that can falter over time. Issues like loose wiring connections, low battery power, electrical shorts, or even just normal wear and tear on parts like sensors can prevent a thermostat from working correctly. If the thermostat itself is damaged or defective, it simply won’t be able to send the proper signals to your HVAC system to shut off when needed.

Clogged Air Filters

One of the most common causes of an AC system that won’t shut off is a soily air filter. Your system relies on sufficient air flow across the AC coil in order to operate properly. When air filters become clogged with dust, pet hair and other debris, it restricts airflow through the system. The system then has trouble reaching the target temperature set on the thermostat and continues running even when it should turn off.

Refrigerant Issues

Problems with the refrigerant in your AC system can also prevent the system from shutting off appropriately. Refrigerant is the substance in your AC that allows it to produce cool air. If the refrigerant level is too low due to a leak, the system will have trouble reaching the target temperature. The thermostat senses that the home is still too warm and keeps telling the AC unit to remain on. An overcharge of refrigerant can cause similar issues.

Faulty Relay Switch

Many AC systems have a relay switch that acts as the go-between to turn the unit on or off based on signals from the thermostat. If this relay fails or gets stuck in one position, it could force the AC unit to remain on even after the thermostat tells it to turn off. The relay may need to be repaired or replaced in this case.

Malfunctioning Contactor

In some AC units, a contactor works together with the thermostat to turn the compressor and fan on and off. If the contactor is damaged or defective, it may not be responding to instructions from the thermostat properly. This can make the AC run continuously.

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Compressor Issues

Problems with the AC compressor itself can also lead to a unit that won’t shut off. Issues like an overheating compressor, failing capacitor or even a seizing compressor can all cause the system to run continuously without cycling on and off as needed. This type of issue requires professional service to diagnose and repair the compressor.

Short Cycling

Sometimes an AC unit will turn on and off too frequently in a short period of time, which is called short cycling. This can happen when the system is undersized for the space or there are airflow restrictions. The thermostat senses the home warming up very quickly after the AC turns off and tells it to come back on after only a few minutes. This makes it seem like the AC is running non-stop.

Troubleshooting Tips for a Thermostat Stuck on AC

troubleshooting tips for a thermostat stuck on ac
Troubleshooting Tips for a Thermostat Stuck on AC

If your thermostat isn’t properly shutting down your air conditioner when the temperature is reached, try these troubleshooting steps:

  • Check thermostat settings – Make sure your thermostat is set to the cooling mode and that the temperature is set higher than the current room temperature. The AC should not run when set above room temp.
  • Change thermostat batteries – Thermostats often have small batteries that provide backup power in case of an outage. Change out dead batteries or place rechargeable ones on the charger.
  • Inspect air filter – Remove air filters and check for blockages. Clogged filters restrict airflow and prevent the AC from cooling effectively. Replace or clean filters as needed.
  • Clean thermostat – Use a soft cloth to gently wipe down the thermostat display and casing. Dust or soil on sensors can interfere with operation.
  • Check wiring connections – Make sure wires running from your thermostat to the AC unit are tightly connected. Loose wiring can stop electrical signals.
  • Test voltage – Use a multimeter to check voltage at the thermostat wiring terminals to confirm power is getting to the device.
  • Watch operation – Observe your system for a while to see if the compressor is actually turning on and off with the thermostat changes.
  • Call an HVAC technician – If self-troubleshooting fails to reveal the issue, it’s best to call in a professional HVAC company to diagnose and repair the problem.

Professional Repairs That May Be Needed

While some thermostat problems can be resolved with minor DIY repairs, there are times when professional expertise is required:

  • Thermostat replacement – If your thermostat is defective and can’t be fixed, a technician can install a suitable replacement with all necessary wiring intact.
  • Refrigerant recharge – If your AC unit is low on refrigerant, identifying any leaks and recharging the refrigerant to proper levels is best left to the pros.
  • Compressor repair – Issues with AC compressors typically require specialized tools and knowledge to diagnose and fix correctly. Let a skilled technician handle compressor problems.
  • Contactor or relay replacement – Faulty contactors or relays that cause continuous AC operation should be replaced by a technician to ensure proper installation.
  • Ductwork repair – If improper ductwork installation is causing short cycling, an HVAC pro can rework ducts for smoother, more balanced airflow.
  • Filter slot modifications – Enlarging tight filter slots can help prevent future clogging and AC issues down the road.
  • New AC unit – In some cases where the AC system is too old or damaged to repair cost effectively, your best option may be to replace the entire unit.

Preventative Maintenance for your AC

To help avoid thermostat and AC problems in the future, be sure to practice regular maintenance:

  • Change air filters monthly during peak usage seasons
  • Have an HVAC technician do annual pre-season tune-ups
  • Clean thermostat screen and housing periodically
  • Check wires and connections to ensure they are tight
  • Clear debris and vegetation away from the outdoor AC unit
  • Replace thermostat batteries yearly
  • Monitor refrigerant levels and recharge as needed
  • Consider installing a smart thermostat for convenience and energy savings
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With a little preventative work, you can hopefully avoid annoying issues like a thermostat that refuses to shut off your air conditioner when it’s supposed to. Follow these troubleshooting tips and consider professional repairs anytime your AC system is not functioning properly. Stay cool and save energy all summer long with a properly operating thermostat.

What To Do if Your New Thermostat Won’t Turn Off AC

You just replaced your old thermostat in hopes of gaining more control over your home’s temperature and energy usage. But despite programming the new thermostat and setting it to higher temperatures, the air conditioner keeps running non-stop. This is understandably frustrating! Before you remove the device and revert back to the old one, let’s look at some quick troubleshooting steps when your new thermostat won’t turn off the AC.

Make Sure It’s Installed Correctly

It seems obvious, but double check that your new thermostat is installed properly. Refer to the manufacturer’s instructions to confirm correct wiring between the thermostat and your HVAC system. Most systems require a C wire to power a digital thermostat. Make sure this common power wire is connected.

Check Thermostat Settings

Review the thermostat display settings to confirm you have it in cooling mode and that the temperature is set above the current room temperature. You can even try turning the setting up significantly higher as a test – if it’s 80 degrees and you set it to 85 or 90, the AC should stop running.

Update Software

Newer digital or smart thermostats may require software updates periodically to fix bugs and improve functionality. If the device won’t turn off the AC, downloading the latest software update from the manufacturer could resolve the problem.

Replace Batteries

Low batteries in a digital thermostat can cause all sorts of issues with temperature regulation and communicating to the AC system. Replace batteries or plug in the device for charging to see if that fixes any software glitches.

Give It Time

Be aware that it can take some time – often 10-15 minutes – for a new thermostat to accurately read the room temperature when first installed. Have patience and let it run for a bit before assuming there’s an issue. The AC may run longer than expected at first as it reaches equilibrium.

Check Sensors

With a smart thermostat, there are often multiple sensors that read temperature in the room and at the device. If one of these sensors is malfunctioning, it could cause the thermostat to misread the room temp. Check for any displayed sensor errors.

Call Support

Many companies offer customer support for thermostat installation and troubleshooting. If you still can’t get a new thermostat to stop running the AC, contact technical support for additional help fixing the issue. Most support teams are quite knowledgeable for their products.

Be patient when installing a new thermostat and follow troubleshooting steps if it won’t turn off your AC appropriately. With a few simple checks, you can typically get the device working properly to deliver comfort and energy savings.

Why Does My Thermostat Keep Turning On the AC?

why does my thermostat keep turning on the ac
Why Does My Thermostat Keep Turning On the AC?

You’ve set your thermostat to a normal temperature that should maintain comfort in your home. But it seems like the air conditioner keeps turning on unnecessarily every few minutes, even when it doesn’t feel hot. What gives?

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There are a few possible culprits for an AC that activates too frequently despite proper thermostat settings:

Short cycling – If your system turns on and off multiple times per hour, it’s likely short cycling. This happens when the unit can’t keep up with the cooling demands, often due to improper sizing or lack of sufficient air flow.

Sun exposure – Direct sunlight on the thermostat tricks it into thinking the room is hotter than it really is. Relocate the thermostat away from windows and sunlight.

Drafty location – Placing a thermostat in a drafty spot like near a vent, door or window can cause false hot or cold readings, triggering the AC unnecessarily.

Dirty filter – A clogged, soily air filter restricts airflow and reduces your AC system’s efficiency, making it work harder than required.

Refrigerant leak – Low refrigerant levels mean your AC has to run longer to reach the target temp. Check refrigerant pressure and top it off if needed.

Malfunctioning thermostat – Bad sensors, software errors or a dying thermostat battery can all cause faulty temperature readings and improper AC activation.

Failing components – Issues with the compressor, fans, condenser, evaporator coil or wiring can force the AC unit to run excessively to compensate.

Undersized system – An AC unit too small for your home will struggle to keep up with cooling demands and cycle on more frequently.

If your AC keeps turning on despite a properly set thermostat, have an HVAC technician inspect your system for issues. Correct sizing, maintenance and component function are key to keeping your unit running optimally.

How To Calibrate Your Thermostat for Accuracy

You woke up feeling freezing cold or sweating from heat and go check the thermostat – yet it’s showing the temperature at a reasonable level. If your thermostat isn’t properly calibrated, it can inaccurately control your home’s heating and cooling. Here’s how to calibrate it for precise operation.

  1. Reset the thermostat – Many models have a calibration feature in settings, or a small hole you insert a pin into to reset to factory settings. Consult manufacturer guidelines for your model.
  2. Give it time to adjust – Let the thermostat sit untouched for 8-12 hours after a reset for the hardware to normalize and adjust itself.
  3. Get an accurate room reading – Use a standalone thermometer in the same location as your thermostat to determine the true current room temperature.
  4. Adjust the thermostat setting – In the settings, look for an option to manually adjust the displayed temperature reading up or down to match your accurate room reading.
  5. Check thermostat after calibration – Allow the thermostat to run for another full day and monitor if it’s turning the AC or heat on and off appropriately based on the calibrated reading.
  6. Continue monitoring performance – Note thermostat accuracy over the next week at different times of day and seasons. Recalibrate as needed if you notice it’s off again.

Properly calibrating your thermostat ensures it knows the real temperature of the room and runs your HVAC system exactly when needed for total home comfort. Take time periodically to check and adjust it for peak performance.


Dealing with a thermostat that fails to shut off your air conditioner can certainly be aggravating and costly on your utility bills. In many cases, some diligent troubleshooting and DIY maintenance can identify and resolve common problems like soily filters, wiring issues or improperly calibrated devices. But for more complex repairs involving refrigerant, compressors or replacement parts, don’t hesitate to call in a trained HVAC technician. With their specialized tools and expertise, professionals can accurately diagnose any issue with your AC and thermostat malfunction and get your system back to running smoothly again. Stop wasting money cooling an empty home – get your thermostat working properly again with a few easy troubleshooting tips or a service call when needed.

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